| .TH PCRE2PATTERN 3 "04 September 2018" "PCRE2 10.32" |
| .SH NAME |
| PCRE2 - Perl-compatible regular expressions (revised API) |
| .SH "PCRE2 REGULAR EXPRESSION DETAILS" |
| .rs |
| .sp |
| The syntax and semantics of the regular expressions that are supported by PCRE2 |
| are described in detail below. There is a quick-reference syntax summary in the |
| .\" HREF |
| \fBpcre2syntax\fP |
| .\" |
| page. PCRE2 tries to match Perl syntax and semantics as closely as it can. |
| PCRE2 also supports some alternative regular expression syntax (which does not |
| conflict with the Perl syntax) in order to provide some compatibility with |
| regular expressions in Python, .NET, and Oniguruma. |
| .P |
| Perl's regular expressions are described in its own documentation, and regular |
| expressions in general are covered in a number of books, some of which have |
| copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published |
| by O'Reilly, covers regular expressions in great detail. This description of |
| PCRE2's regular expressions is intended as reference material. |
| .P |
| This document discusses the patterns that are supported by PCRE2 when its main |
| matching function, \fBpcre2_match()\fP, is used. PCRE2 also has an alternative |
| matching function, \fBpcre2_dfa_match()\fP, which matches using a different |
| algorithm that is not Perl-compatible. Some of the features discussed below are |
| not available when DFA matching is used. The advantages and disadvantages of |
| the alternative function, and how it differs from the normal function, are |
| discussed in the |
| .\" HREF |
| \fBpcre2matching\fP |
| .\" |
| page. |
| . |
| . |
| .SH "SPECIAL START-OF-PATTERN ITEMS" |
| .rs |
| .sp |
| A number of options that can be passed to \fBpcre2_compile()\fP can also be set |
| by special items at the start of a pattern. These are not Perl-compatible, but |
| are provided to make these options accessible to pattern writers who are not |
| able to change the program that processes the pattern. Any number of these |
| items may appear, but they must all be together right at the start of the |
| pattern string, and the letters must be in upper case. |
| . |
| . |
| .SS "UTF support" |
| .rs |
| .sp |
| In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either as |
| single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32 can be |
| specified for the 32-bit library, in which case it constrains the character |
| values to valid Unicode code points. To process UTF strings, PCRE2 must be |
| built to include Unicode support (which is the default). When using UTF strings |
| you must either call the compiling function with the PCRE2_UTF option, or the |
| pattern must start with the special sequence (*UTF), which is equivalent to |
| setting the relevant option. How setting a UTF mode affects pattern matching is |
| mentioned in several places below. There is also a summary of features in the |
| .\" HREF |
| \fBpcre2unicode\fP |
| .\" |
| page. |
| .P |
| Some applications that allow their users to supply patterns may wish to |
| restrict them to non-UTF data for security reasons. If the PCRE2_NEVER_UTF |
| option is passed to \fBpcre2_compile()\fP, (*UTF) is not allowed, and its |
| appearance in a pattern causes an error. |
| . |
| . |
| .SS "Unicode property support" |
| .rs |
| .sp |
| Another special sequence that may appear at the start of a pattern is (*UCP). |
| This has the same effect as setting the PCRE2_UCP option: it causes sequences |
| such as \ed and \ew to use Unicode properties to determine character types, |
| instead of recognizing only characters with codes less than 256 via a lookup |
| table. |
| .P |
| Some applications that allow their users to supply patterns may wish to |
| restrict them for security reasons. If the PCRE2_NEVER_UCP option is passed to |
| \fBpcre2_compile()\fP, (*UCP) is not allowed, and its appearance in a pattern |
| causes an error. |
| . |
| . |
| .SS "Locking out empty string matching" |
| .rs |
| .sp |
| Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same effect |
| as passing the PCRE2_NOTEMPTY or PCRE2_NOTEMPTY_ATSTART option to whichever |
| matching function is subsequently called to match the pattern. These options |
| lock out the matching of empty strings, either entirely, or only at the start |
| of the subject. |
| . |
| . |
| .SS "Disabling auto-possessification" |
| .rs |
| .sp |
| If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting |
| the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from making quantifiers |
| possessive when what follows cannot match the repeated item. For example, by |
| default a+b is treated as a++b. For more details, see the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| documentation. |
| . |
| . |
| .SS "Disabling start-up optimizations" |
| .rs |
| .sp |
| If a pattern starts with (*NO_START_OPT), it has the same effect as setting the |
| PCRE2_NO_START_OPTIMIZE option. This disables several optimizations for quickly |
| reaching "no match" results. For more details, see the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| documentation. |
| . |
| . |
| .SS "Disabling automatic anchoring" |
| .rs |
| .sp |
| If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the same effect as |
| setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimizations that |
| apply to patterns whose top-level branches all start with .* (match any number |
| of arbitrary characters). For more details, see the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| documentation. |
| . |
| . |
| .SS "Disabling JIT compilation" |
| .rs |
| .sp |
| If a pattern that starts with (*NO_JIT) is successfully compiled, an attempt by |
| the application to apply the JIT optimization by calling |
| \fBpcre2_jit_compile()\fP is ignored. |
| . |
| . |
| .SS "Setting match resource limits" |
| .rs |
| .sp |
| The \fBpcre2_match()\fP function contains a counter that is incremented every |
| time it goes round its main loop. The caller of \fBpcre2_match()\fP can set a |
| limit on this counter, which therefore limits the amount of computing resource |
| used for a match. The maximum depth of nested backtracking can also be limited; |
| this indirectly restricts the amount of heap memory that is used, but there is |
| also an explicit memory limit that can be set. |
| .P |
| These facilities are provided to catch runaway matches that are provoked by |
| patterns with huge matching trees (a typical example is a pattern with nested |
| unlimited repeats applied to a long string that does not match). When one of |
| these limits is reached, \fBpcre2_match()\fP gives an error return. The limits |
| can also be set by items at the start of the pattern of the form |
| .sp |
| (*LIMIT_HEAP=d) |
| (*LIMIT_MATCH=d) |
| (*LIMIT_DEPTH=d) |
| .sp |
| where d is any number of decimal digits. However, the value of the setting must |
| be less than the value set (or defaulted) by the caller of \fBpcre2_match()\fP |
| for it to have any effect. In other words, the pattern writer can lower the |
| limits set by the programmer, but not raise them. If there is more than one |
| setting of one of these limits, the lower value is used. The heap limit is |
| specified in kibibytes (units of 1024 bytes). |
| .P |
| Prior to release 10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This name is |
| still recognized for backwards compatibility. |
| .P |
| The heap limit applies only when the \fBpcre2_match()\fP or |
| \fBpcre2_dfa_match()\fP interpreters are used for matching. It does not apply |
| to JIT. The match limit is used (but in a different way) when JIT is being |
| used, or when \fBpcre2_dfa_match()\fP is called, to limit computing resource |
| usage by those matching functions. The depth limit is ignored by JIT but is |
| relevant for DFA matching, which uses function recursion for recursions within |
| the pattern and for lookaround assertions and atomic groups. In this case, the |
| depth limit controls the depth of such recursion. |
| . |
| . |
| .\" HTML <a name="newlines"></a> |
| .SS "Newline conventions" |
| .rs |
| .sp |
| PCRE2 supports six different conventions for indicating line breaks in |
| strings: a single CR (carriage return) character, a single LF (linefeed) |
| character, the two-character sequence CRLF, any of the three preceding, any |
| Unicode newline sequence, or the NUL character (binary zero). The |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| page has |
| .\" HTML <a href="pcre2api.html#newlines"> |
| .\" </a> |
| further discussion |
| .\" |
| about newlines, and shows how to set the newline convention when calling |
| \fBpcre2_compile()\fP. |
| .P |
| It is also possible to specify a newline convention by starting a pattern |
| string with one of the following sequences: |
| .sp |
| (*CR) carriage return |
| (*LF) linefeed |
| (*CRLF) carriage return, followed by linefeed |
| (*ANYCRLF) any of the three above |
| (*ANY) all Unicode newline sequences |
| (*NUL) the NUL character (binary zero) |
| .sp |
| These override the default and the options given to the compiling function. For |
| example, on a Unix system where LF is the default newline sequence, the pattern |
| .sp |
| (*CR)a.b |
| .sp |
| changes the convention to CR. That pattern matches "a\enb" because LF is no |
| longer a newline. If more than one of these settings is present, the last one |
| is used. |
| .P |
| The newline convention affects where the circumflex and dollar assertions are |
| true. It also affects the interpretation of the dot metacharacter when |
| PCRE2_DOTALL is not set, and the behaviour of \eN when not followed by an |
| opening brace. However, it does not affect what the \eR escape sequence |
| matches. By default, this is any Unicode newline sequence, for Perl |
| compatibility. However, this can be changed; see the next section and the |
| description of \eR in the section entitled |
| .\" HTML <a href="#newlineseq"> |
| .\" </a> |
| "Newline sequences" |
| .\" |
| below. A change of \eR setting can be combined with a change of newline |
| convention. |
| . |
| . |
| .SS "Specifying what \eR matches" |
| .rs |
| .sp |
| It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the |
| complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF |
| at compile time. This effect can also be achieved by starting a pattern with |
| (*BSR_ANYCRLF). For completeness, (*BSR_UNICODE) is also recognized, |
| corresponding to PCRE2_BSR_UNICODE. |
| . |
| . |
| .SH "EBCDIC CHARACTER CODES" |
| .rs |
| .sp |
| PCRE2 can be compiled to run in an environment that uses EBCDIC as its |
| character code instead of ASCII or Unicode (typically a mainframe system). In |
| the sections below, character code values are ASCII or Unicode; in an EBCDIC |
| environment these characters may have different code values, and there are no |
| code points greater than 255. |
| . |
| . |
| .SH "CHARACTERS AND METACHARACTERS" |
| .rs |
| .sp |
| A regular expression is a pattern that is matched against a subject string from |
| left to right. Most characters stand for themselves in a pattern, and match the |
| corresponding characters in the subject. As a trivial example, the pattern |
| .sp |
| The quick brown fox |
| .sp |
| matches a portion of a subject string that is identical to itself. When |
| caseless matching is specified (the PCRE2_CASELESS option), letters are matched |
| independently of case. |
| .P |
| The power of regular expressions comes from the ability to include alternatives |
| and repetitions in the pattern. These are encoded in the pattern by the use of |
| \fImetacharacters\fP, which do not stand for themselves but instead are |
| interpreted in some special way. |
| .P |
| There are two different sets of metacharacters: those that are recognized |
| anywhere in the pattern except within square brackets, and those that are |
| recognized within square brackets. Outside square brackets, the metacharacters |
| are as follows: |
| .sp |
| \e general escape character with several uses |
| ^ assert start of string (or line, in multiline mode) |
| $ assert end of string (or line, in multiline mode) |
| . match any character except newline (by default) |
| [ start character class definition |
| | start of alternative branch |
| ( start subpattern |
| ) end subpattern |
| ? extends the meaning of ( |
| also 0 or 1 quantifier |
| also quantifier minimizer |
| * 0 or more quantifier |
| + 1 or more quantifier |
| also "possessive quantifier" |
| { start min/max quantifier |
| .sp |
| Part of a pattern that is in square brackets is called a "character class". In |
| a character class the only metacharacters are: |
| .sp |
| \e general escape character |
| ^ negate the class, but only if the first character |
| - indicates character range |
| .\" JOIN |
| [ POSIX character class (only if followed by POSIX |
| syntax) |
| ] terminates the character class |
| .sp |
| The following sections describe the use of each of the metacharacters. |
| . |
| . |
| .SH BACKSLASH |
| .rs |
| .sp |
| The backslash character has several uses. Firstly, if it is followed by a |
| character that is not a number or a letter, it takes away any special meaning |
| that character may have. This use of backslash as an escape character applies |
| both inside and outside character classes. |
| .P |
| For example, if you want to match a * character, you must write \e* in the |
| pattern. This escaping action applies whether or not the following character |
| would otherwise be interpreted as a metacharacter, so it is always safe to |
| precede a non-alphanumeric with backslash to specify that it stands for itself. |
| In particular, if you want to match a backslash, you write \e\e. |
| .P |
| In a UTF mode, only ASCII numbers and letters have any special meaning after a |
| backslash. All other characters (in particular, those whose code points are |
| greater than 127) are treated as literals. |
| .P |
| If a pattern is compiled with the PCRE2_EXTENDED option, most white space in |
| the pattern (other than in a character class), and characters between a # |
| outside a character class and the next newline, inclusive, are ignored. An |
| escaping backslash can be used to include a white space or # character as part |
| of the pattern. |
| .P |
| If you want to remove the special meaning from a sequence of characters, you |
| can do so by putting them between \eQ and \eE. This is different from Perl in |
| that $ and @ are handled as literals in \eQ...\eE sequences in PCRE2, whereas |
| in Perl, $ and @ cause variable interpolation. Also, Perl does "double-quotish |
| backslash interpolation" on any backslashes between \eQ and \eE which, its |
| documentation says, "may lead to confusing results". PCRE2 treats a backslash |
| between \eQ and \eE just like any other character. Note the following examples: |
| .sp |
| Pattern PCRE2 matches Perl matches |
| .sp |
| .\" JOIN |
| \eQabc$xyz\eE abc$xyz abc followed by the |
| contents of $xyz |
| \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz |
| \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz |
| \eQA\eB\eE A\eB A\eB |
| \eQ\e\eE \e \e\eE |
| .sp |
| The \eQ...\eE sequence is recognized both inside and outside character classes. |
| An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed |
| by \eE later in the pattern, the literal interpretation continues to the end of |
| the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside |
| a character class, this causes an error, because the character class is not |
| terminated by a closing square bracket. |
| . |
| . |
| .\" HTML <a name="digitsafterbackslash"></a> |
| .SS "Non-printing characters" |
| .rs |
| .sp |
| A second use of backslash provides a way of encoding non-printing characters |
| in patterns in a visible manner. There is no restriction on the appearance of |
| non-printing characters in a pattern, but when a pattern is being prepared by |
| text editing, it is often easier to use one of the following escape sequences |
| than the binary character it represents. In an ASCII or Unicode environment, |
| these escapes are as follows: |
| .sp |
| \ea alarm, that is, the BEL character (hex 07) |
| \ecx "control-x", where x is any printable ASCII character |
| \ee escape (hex 1B) |
| \ef form feed (hex 0C) |
| \en linefeed (hex 0A) |
| \er carriage return (hex 0D) |
| \et tab (hex 09) |
| \e0dd character with octal code 0dd |
| \eddd character with octal code ddd, or backreference |
| \eo{ddd..} character with octal code ddd.. |
| \exhh character with hex code hh |
| \ex{hhh..} character with hex code hhh.. |
| \eN{U+hhh..} character with Unicode hex code point hhh.. |
| \euhhhh character with hex code hhhh (when PCRE2_ALT_BSUX is set) |
| .sp |
| The \eN{U+hhh..} escape sequence is recognized only when the PCRE2_UTF option |
| is set, that is, when PCRE2 is operating in a Unicode mode. Perl also uses |
| \eN{name} to specify characters by Unicode name; PCRE2 does not support this. |
| Note that when \eN is not followed by an opening brace (curly bracket) it has |
| an entirely different meaning, matching any character that is not a newline. |
| .P |
| The precise effect of \ecx on ASCII characters is as follows: if x is a lower |
| case letter, it is converted to upper case. Then bit 6 of the character (hex |
| 40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A), |
| but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the |
| code unit following \ec has a value less than 32 or greater than 126, a |
| compile-time error occurs. |
| .P |
| When PCRE2 is compiled in EBCDIC mode, \eN{U+hhh..} is not supported. \ea, \ee, |
| \ef, \en, \er, and \et generate the appropriate EBCDIC code values. The \ec |
| escape is processed as specified for Perl in the \fBperlebcdic\fP document. The |
| only characters that are allowed after \ec are A-Z, a-z, or one of @, [, \e, ], |
| ^, _, or ?. Any other character provokes a compile-time error. The sequence |
| \ec@ encodes character code 0; after \ec the letters (in either case) encode |
| characters 1-26 (hex 01 to hex 1A); [, \e, ], ^, and _ encode characters 27-31 |
| (hex 1B to hex 1F), and \ec? becomes either 255 (hex FF) or 95 (hex 5F). |
| .P |
| Thus, apart from \ec?, these escapes generate the same character code values as |
| they do in an ASCII environment, though the meanings of the values mostly |
| differ. For example, \ecG always generates code value 7, which is BEL in ASCII |
| but DEL in EBCDIC. |
| .P |
| The sequence \ec? generates DEL (127, hex 7F) in an ASCII environment, but |
| because 127 is not a control character in EBCDIC, Perl makes it generate the |
| APC character. Unfortunately, there are several variants of EBCDIC. In most of |
| them the APC character has the value 255 (hex FF), but in the one Perl calls |
| POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC |
| values, PCRE2 makes \ec? generate 95; otherwise it generates 255. |
| .P |
| After \e0 up to two further octal digits are read. If there are fewer than two |
| digits, just those that are present are used. Thus the sequence \e0\ex\e015 |
| specifies two binary zeros followed by a CR character (code value 13). Make |
| sure you supply two digits after the initial zero if the pattern character that |
| follows is itself an octal digit. |
| .P |
| The escape \eo must be followed by a sequence of octal digits, enclosed in |
| braces. An error occurs if this is not the case. This escape is a recent |
| addition to Perl; it provides way of specifying character code points as octal |
| numbers greater than 0777, and it also allows octal numbers and backreferences |
| to be unambiguously specified. |
| .P |
| For greater clarity and unambiguity, it is best to avoid following \e by a |
| digit greater than zero. Instead, use \eo{} or \ex{} to specify numerical |
| character code points, and \eg{} to specify backreferences. The following |
| paragraphs describe the old, ambiguous syntax. |
| .P |
| The handling of a backslash followed by a digit other than 0 is complicated, |
| and Perl has changed over time, causing PCRE2 also to change. |
| .P |
| Outside a character class, PCRE2 reads the digit and any following digits as a |
| decimal number. If the number is less than 10, begins with the digit 8 or 9, or |
| if there are at least that many previous capturing left parentheses in the |
| expression, the entire sequence is taken as a \fIbackreference\fP. A |
| description of how this works is given |
| .\" HTML <a href="#backreferences"> |
| .\" </a> |
| later, |
| .\" |
| following the discussion of |
| .\" HTML <a href="#subpattern"> |
| .\" </a> |
| parenthesized subpatterns. |
| .\" |
| Otherwise, up to three octal digits are read to form a character code. |
| .P |
| Inside a character class, PCRE2 handles \e8 and \e9 as the literal characters |
| "8" and "9", and otherwise reads up to three octal digits following the |
| backslash, using them to generate a data character. Any subsequent digits stand |
| for themselves. For example, outside a character class: |
| .sp |
| \e040 is another way of writing an ASCII space |
| .\" JOIN |
| \e40 is the same, provided there are fewer than 40 |
| previous capturing subpatterns |
| \e7 is always a backreference |
| .\" JOIN |
| \e11 might be a backreference, or another way of |
| writing a tab |
| \e011 is always a tab |
| \e0113 is a tab followed by the character "3" |
| .\" JOIN |
| \e113 might be a backreference, otherwise the |
| character with octal code 113 |
| .\" JOIN |
| \e377 might be a backreference, otherwise |
| the value 255 (decimal) |
| .\" JOIN |
| \e81 is always a backreference |
| .sp |
| Note that octal values of 100 or greater that are specified using this syntax |
| must not be introduced by a leading zero, because no more than three octal |
| digits are ever read. |
| .P |
| By default, after \ex that is not followed by {, from zero to two hexadecimal |
| digits are read (letters can be in upper or lower case). Any number of |
| hexadecimal digits may appear between \ex{ and }. If a character other than |
| a hexadecimal digit appears between \ex{ and }, or if there is no terminating |
| }, an error occurs. |
| .P |
| If the PCRE2_ALT_BSUX option is set, the interpretation of \ex is as just |
| described only when it is followed by two hexadecimal digits. Otherwise, it |
| matches a literal "x" character. In this mode, support for code points greater |
| than 256 is provided by \eu, which must be followed by four hexadecimal digits; |
| otherwise it matches a literal "u" character. |
| .P |
| Characters whose value is less than 256 can be defined by either of the two |
| syntaxes for \ex (or by \eu in PCRE2_ALT_BSUX mode). There is no difference in |
| the way they are handled. For example, \exdc is exactly the same as \ex{dc} (or |
| \eu00dc in PCRE2_ALT_BSUX mode). |
| . |
| . |
| .SS "Constraints on character values" |
| .rs |
| .sp |
| Characters that are specified using octal or hexadecimal numbers are |
| limited to certain values, as follows: |
| .sp |
| 8-bit non-UTF mode no greater than 0xff |
| 16-bit non-UTF mode no greater than 0xffff |
| 32-bit non-UTF mode no greater than 0xffffffff |
| All UTF modes no greater than 0x10ffff and a valid code point |
| .sp |
| Invalid Unicode code points are all those in the range 0xd800 to 0xdfff (the |
| so-called "surrogate" code points). The check for these can be disabled by the |
| caller of \fBpcre2_compile()\fP by setting the option |
| PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES. However, this is possible only in UTF-8 |
| and UTF-32 modes, because these values are not representable in UTF-16. |
| . |
| . |
| .SS "Escape sequences in character classes" |
| .rs |
| .sp |
| All the sequences that define a single character value can be used both inside |
| and outside character classes. In addition, inside a character class, \eb is |
| interpreted as the backspace character (hex 08). |
| .P |
| When not followed by an opening brace, \eN is not allowed in a character class. |
| \eB, \eR, and \eX are not special inside a character class. Like other |
| unrecognized alphabetic escape sequences, they cause an error. Outside a |
| character class, these sequences have different meanings. |
| . |
| . |
| .SS "Unsupported escape sequences" |
| .rs |
| .sp |
| In Perl, the sequences \eF, \el, \eL, \eu, and \eU are recognized by its string |
| handler and used to modify the case of following characters. By default, PCRE2 |
| does not support these escape sequences. However, if the PCRE2_ALT_BSUX option |
| is set, \eU matches a "U" character, and \eu can be used to define a character |
| by code point, as described above. |
| . |
| . |
| .SS "Absolute and relative backreferences" |
| .rs |
| .sp |
| The sequence \eg followed by a signed or unsigned number, optionally enclosed |
| in braces, is an absolute or relative backreference. A named backreference |
| can be coded as \eg{name}. Backreferences are discussed |
| .\" HTML <a href="#backreferences"> |
| .\" </a> |
| later, |
| .\" |
| following the discussion of |
| .\" HTML <a href="#subpattern"> |
| .\" </a> |
| parenthesized subpatterns. |
| .\" |
| . |
| . |
| .SS "Absolute and relative subroutine calls" |
| .rs |
| .sp |
| For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or |
| a number enclosed either in angle brackets or single quotes, is an alternative |
| syntax for referencing a subpattern as a "subroutine". Details are discussed |
| .\" HTML <a href="#onigurumasubroutines"> |
| .\" </a> |
| later. |
| .\" |
| Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP |
| synonymous. The former is a backreference; the latter is a |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| subroutine |
| .\" |
| call. |
| . |
| . |
| .\" HTML <a name="genericchartypes"></a> |
| .SS "Generic character types" |
| .rs |
| .sp |
| Another use of backslash is for specifying generic character types: |
| .sp |
| \ed any decimal digit |
| \eD any character that is not a decimal digit |
| \eh any horizontal white space character |
| \eH any character that is not a horizontal white space character |
| \eN any character that is not a newline |
| \es any white space character |
| \eS any character that is not a white space character |
| \ev any vertical white space character |
| \eV any character that is not a vertical white space character |
| \ew any "word" character |
| \eW any "non-word" character |
| .sp |
| The \eN escape sequence has the same meaning as |
| .\" HTML <a href="#fullstopdot"> |
| .\" </a> |
| the "." metacharacter |
| .\" |
| when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change the |
| meaning of \eN. Note that when \eN is followed by an opening brace it has a |
| different meaning. See the section entitled |
| .\" HTML <a href="#digitsafterbackslash"> |
| .\" </a> |
| "Non-printing characters" |
| .\" |
| above for details. Perl also uses \eN{name} to specify characters by Unicode |
| name; PCRE2 does not support this. |
| .P |
| Each pair of lower and upper case escape sequences partitions the complete set |
| of characters into two disjoint sets. Any given character matches one, and only |
| one, of each pair. The sequences can appear both inside and outside character |
| classes. They each match one character of the appropriate type. If the current |
| matching point is at the end of the subject string, all of them fail, because |
| there is no character to match. |
| .P |
| The default \es characters are HT (9), LF (10), VT (11), FF (12), CR (13), and |
| space (32), which are defined as white space in the "C" locale. This list may |
| vary if locale-specific matching is taking place. For example, in some locales |
| the "non-breaking space" character (\exA0) is recognized as white space, and in |
| others the VT character is not. |
| .P |
| A "word" character is an underscore or any character that is a letter or digit. |
| By default, the definition of letters and digits is controlled by PCRE2's |
| low-valued character tables, and may vary if locale-specific matching is taking |
| place (see |
| .\" HTML <a href="pcre2api.html#localesupport"> |
| .\" </a> |
| "Locale support" |
| .\" |
| in the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| page). For example, in a French locale such as "fr_FR" in Unix-like systems, |
| or "french" in Windows, some character codes greater than 127 are used for |
| accented letters, and these are then matched by \ew. The use of locales with |
| Unicode is discouraged. |
| .P |
| By default, characters whose code points are greater than 127 never match \ed, |
| \es, or \ew, and always match \eD, \eS, and \eW, although this may be different |
| for characters in the range 128-255 when locale-specific matching is happening. |
| These escape sequences retain their original meanings from before Unicode |
| support was available, mainly for efficiency reasons. If the PCRE2_UCP option |
| is set, the behaviour is changed so that Unicode properties are used to |
| determine character types, as follows: |
| .sp |
| \ed any character that matches \ep{Nd} (decimal digit) |
| \es any character that matches \ep{Z} or \eh or \ev |
| \ew any character that matches \ep{L} or \ep{N}, plus underscore |
| .sp |
| The upper case escapes match the inverse sets of characters. Note that \ed |
| matches only decimal digits, whereas \ew matches any Unicode digit, as well as |
| any Unicode letter, and underscore. Note also that PCRE2_UCP affects \eb, and |
| \eB because they are defined in terms of \ew and \eW. Matching these sequences |
| is noticeably slower when PCRE2_UCP is set. |
| .P |
| The sequences \eh, \eH, \ev, and \eV, in contrast to the other sequences, which |
| match only ASCII characters by default, always match a specific list of code |
| points, whether or not PCRE2_UCP is set. The horizontal space characters are: |
| .sp |
| U+0009 Horizontal tab (HT) |
| U+0020 Space |
| U+00A0 Non-break space |
| U+1680 Ogham space mark |
| U+180E Mongolian vowel separator |
| U+2000 En quad |
| U+2001 Em quad |
| U+2002 En space |
| U+2003 Em space |
| U+2004 Three-per-em space |
| U+2005 Four-per-em space |
| U+2006 Six-per-em space |
| U+2007 Figure space |
| U+2008 Punctuation space |
| U+2009 Thin space |
| U+200A Hair space |
| U+202F Narrow no-break space |
| U+205F Medium mathematical space |
| U+3000 Ideographic space |
| .sp |
| The vertical space characters are: |
| .sp |
| U+000A Linefeed (LF) |
| U+000B Vertical tab (VT) |
| U+000C Form feed (FF) |
| U+000D Carriage return (CR) |
| U+0085 Next line (NEL) |
| U+2028 Line separator |
| U+2029 Paragraph separator |
| .sp |
| In 8-bit, non-UTF-8 mode, only the characters with code points less than 256 |
| are relevant. |
| . |
| . |
| .\" HTML <a name="newlineseq"></a> |
| .SS "Newline sequences" |
| .rs |
| .sp |
| Outside a character class, by default, the escape sequence \eR matches any |
| Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the |
| following: |
| .sp |
| (?>\er\en|\en|\ex0b|\ef|\er|\ex85) |
| .sp |
| This is an example of an "atomic group", details of which are given |
| .\" HTML <a href="#atomicgroup"> |
| .\" </a> |
| below. |
| .\" |
| This particular group matches either the two-character sequence CR followed by |
| LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, |
| U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next |
| line, U+0085). Because this is an atomic group, the two-character sequence is |
| treated as a single unit that cannot be split. |
| .P |
| In other modes, two additional characters whose code points are greater than 255 |
| are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). |
| Unicode support is not needed for these characters to be recognized. |
| .P |
| It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the |
| complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF |
| at compile time. (BSR is an abbrevation for "backslash R".) This can be made |
| the default when PCRE2 is built; if this is the case, the other behaviour can |
| be requested via the PCRE2_BSR_UNICODE option. It is also possible to specify |
| these settings by starting a pattern string with one of the following |
| sequences: |
| .sp |
| (*BSR_ANYCRLF) CR, LF, or CRLF only |
| (*BSR_UNICODE) any Unicode newline sequence |
| .sp |
| These override the default and the options given to the compiling function. |
| Note that these special settings, which are not Perl-compatible, are recognized |
| only at the very start of a pattern, and that they must be in upper case. If |
| more than one of them is present, the last one is used. They can be combined |
| with a change of newline convention; for example, a pattern can start with: |
| .sp |
| (*ANY)(*BSR_ANYCRLF) |
| .sp |
| They can also be combined with the (*UTF) or (*UCP) special sequences. Inside a |
| character class, \eR is treated as an unrecognized escape sequence, and causes |
| an error. |
| . |
| . |
| .\" HTML <a name="uniextseq"></a> |
| .SS Unicode character properties |
| .rs |
| .sp |
| When PCRE2 is built with Unicode support (the default), three additional escape |
| sequences that match characters with specific properties are available. In |
| 8-bit non-UTF-8 mode, these sequences are of course limited to testing |
| characters whose code points are less than 256, but they do work in this mode. |
| In 32-bit non-UTF mode, code points greater than 0x10ffff (the Unicode limit) |
| may be encountered. These are all treated as being in the Common script and |
| with an unassigned type. The extra escape sequences are: |
| .sp |
| \ep{\fIxx\fP} a character with the \fIxx\fP property |
| \eP{\fIxx\fP} a character without the \fIxx\fP property |
| \eX a Unicode extended grapheme cluster |
| .sp |
| The property names represented by \fIxx\fP above are limited to the Unicode |
| script names, the general category properties, "Any", which matches any |
| character (including newline), and some special PCRE2 properties (described |
| in the |
| .\" HTML <a href="#extraprops"> |
| .\" </a> |
| next section). |
| .\" |
| Other Perl properties such as "InMusicalSymbols" are not supported by PCRE2. |
| Note that \eP{Any} does not match any characters, so always causes a match |
| failure. |
| .P |
| Sets of Unicode characters are defined as belonging to certain scripts. A |
| character from one of these sets can be matched using a script name. For |
| example: |
| .sp |
| \ep{Greek} |
| \eP{Han} |
| .sp |
| Those that are not part of an identified script are lumped together as |
| "Common". The current list of scripts is: |
| .P |
| Adlam, |
| Ahom, |
| Anatolian_Hieroglyphs, |
| Arabic, |
| Armenian, |
| Avestan, |
| Balinese, |
| Bamum, |
| Bassa_Vah, |
| Batak, |
| Bengali, |
| Bhaiksuki, |
| Bopomofo, |
| Brahmi, |
| Braille, |
| Buginese, |
| Buhid, |
| Canadian_Aboriginal, |
| Carian, |
| Caucasian_Albanian, |
| Chakma, |
| Cham, |
| Cherokee, |
| Common, |
| Coptic, |
| Cuneiform, |
| Cypriot, |
| Cyrillic, |
| Deseret, |
| Devanagari, |
| Dogra, |
| Duployan, |
| Egyptian_Hieroglyphs, |
| Elbasan, |
| Ethiopic, |
| Georgian, |
| Glagolitic, |
| Gothic, |
| Grantha, |
| Greek, |
| Gujarati, |
| Gunjala_Gondi, |
| Gurmukhi, |
| Han, |
| Hangul, |
| Hanifi_Rohingya, |
| Hanunoo, |
| Hatran, |
| Hebrew, |
| Hiragana, |
| Imperial_Aramaic, |
| Inherited, |
| Inscriptional_Pahlavi, |
| Inscriptional_Parthian, |
| Javanese, |
| Kaithi, |
| Kannada, |
| Katakana, |
| Kayah_Li, |
| Kharoshthi, |
| Khmer, |
| Khojki, |
| Khudawadi, |
| Lao, |
| Latin, |
| Lepcha, |
| Limbu, |
| Linear_A, |
| Linear_B, |
| Lisu, |
| Lycian, |
| Lydian, |
| Mahajani, |
| Makasar, |
| Malayalam, |
| Mandaic, |
| Manichaean, |
| Marchen, |
| Masaram_Gondi, |
| Medefaidrin, |
| Meetei_Mayek, |
| Mende_Kikakui, |
| Meroitic_Cursive, |
| Meroitic_Hieroglyphs, |
| Miao, |
| Modi, |
| Mongolian, |
| Mro, |
| Multani, |
| Myanmar, |
| Nabataean, |
| New_Tai_Lue, |
| Newa, |
| Nko, |
| Nushu, |
| Ogham, |
| Ol_Chiki, |
| Old_Hungarian, |
| Old_Italic, |
| Old_North_Arabian, |
| Old_Permic, |
| Old_Persian, |
| Old_Sogdian, |
| Old_South_Arabian, |
| Old_Turkic, |
| Oriya, |
| Osage, |
| Osmanya, |
| Pahawh_Hmong, |
| Palmyrene, |
| Pau_Cin_Hau, |
| Phags_Pa, |
| Phoenician, |
| Psalter_Pahlavi, |
| Rejang, |
| Runic, |
| Samaritan, |
| Saurashtra, |
| Sharada, |
| Shavian, |
| Siddham, |
| SignWriting, |
| Sinhala, |
| Sogdian, |
| Sora_Sompeng, |
| Soyombo, |
| Sundanese, |
| Syloti_Nagri, |
| Syriac, |
| Tagalog, |
| Tagbanwa, |
| Tai_Le, |
| Tai_Tham, |
| Tai_Viet, |
| Takri, |
| Tamil, |
| Tangut, |
| Telugu, |
| Thaana, |
| Thai, |
| Tibetan, |
| Tifinagh, |
| Tirhuta, |
| Ugaritic, |
| Vai, |
| Warang_Citi, |
| Yi, |
| Zanabazar_Square. |
| .P |
| Each character has exactly one Unicode general category property, specified by |
| a two-letter abbreviation. For compatibility with Perl, negation can be |
| specified by including a circumflex between the opening brace and the property |
| name. For example, \ep{^Lu} is the same as \eP{Lu}. |
| .P |
| If only one letter is specified with \ep or \eP, it includes all the general |
| category properties that start with that letter. In this case, in the absence |
| of negation, the curly brackets in the escape sequence are optional; these two |
| examples have the same effect: |
| .sp |
| \ep{L} |
| \epL |
| .sp |
| The following general category property codes are supported: |
| .sp |
| C Other |
| Cc Control |
| Cf Format |
| Cn Unassigned |
| Co Private use |
| Cs Surrogate |
| .sp |
| L Letter |
| Ll Lower case letter |
| Lm Modifier letter |
| Lo Other letter |
| Lt Title case letter |
| Lu Upper case letter |
| .sp |
| M Mark |
| Mc Spacing mark |
| Me Enclosing mark |
| Mn Non-spacing mark |
| .sp |
| N Number |
| Nd Decimal number |
| Nl Letter number |
| No Other number |
| .sp |
| P Punctuation |
| Pc Connector punctuation |
| Pd Dash punctuation |
| Pe Close punctuation |
| Pf Final punctuation |
| Pi Initial punctuation |
| Po Other punctuation |
| Ps Open punctuation |
| .sp |
| S Symbol |
| Sc Currency symbol |
| Sk Modifier symbol |
| Sm Mathematical symbol |
| So Other symbol |
| .sp |
| Z Separator |
| Zl Line separator |
| Zp Paragraph separator |
| Zs Space separator |
| .sp |
| The special property L& is also supported: it matches a character that has |
| the Lu, Ll, or Lt property, in other words, a letter that is not classified as |
| a modifier or "other". |
| .P |
| The Cs (Surrogate) property applies only to characters in the range U+D800 to |
| U+DFFF. Such characters are not valid in Unicode strings and so |
| cannot be tested by PCRE2, unless UTF validity checking has been turned off |
| (see the discussion of PCRE2_NO_UTF_CHECK in the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| page). Perl does not support the Cs property. |
| .P |
| The long synonyms for property names that Perl supports (such as \ep{Letter}) |
| are not supported by PCRE2, nor is it permitted to prefix any of these |
| properties with "Is". |
| .P |
| No character that is in the Unicode table has the Cn (unassigned) property. |
| Instead, this property is assumed for any code point that is not in the |
| Unicode table. |
| .P |
| Specifying caseless matching does not affect these escape sequences. For |
| example, \ep{Lu} always matches only upper case letters. This is different from |
| the behaviour of current versions of Perl. |
| .P |
| Matching characters by Unicode property is not fast, because PCRE2 has to do a |
| multistage table lookup in order to find a character's property. That is why |
| the traditional escape sequences such as \ed and \ew do not use Unicode |
| properties in PCRE2 by default, though you can make them do so by setting the |
| PCRE2_UCP option or by starting the pattern with (*UCP). |
| . |
| . |
| .SS Extended grapheme clusters |
| .rs |
| .sp |
| The \eX escape matches any number of Unicode characters that form an "extended |
| grapheme cluster", and treats the sequence as an atomic group |
| .\" HTML <a href="#atomicgroup"> |
| .\" </a> |
| (see below). |
| .\" |
| Unicode supports various kinds of composite character by giving each character |
| a grapheme breaking property, and having rules that use these properties to |
| define the boundaries of extended grapheme clusters. The rules are defined in |
| Unicode Standard Annex 29, "Unicode Text Segmentation". Unicode 11.0.0 |
| abandoned the use of some previous properties that had been used for emojis. |
| Instead it introduced various emoji-specific properties. PCRE2 uses only the |
| Extended Pictographic property. |
| .P |
| \eX always matches at least one character. Then it decides whether to add |
| additional characters according to the following rules for ending a cluster: |
| .P |
| 1. End at the end of the subject string. |
| .P |
| 2. Do not end between CR and LF; otherwise end after any control character. |
| .P |
| 3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters |
| are of five types: L, V, T, LV, and LVT. An L character may be followed by an |
| L, V, LV, or LVT character; an LV or V character may be followed by a V or T |
| character; an LVT or T character may be follwed only by a T character. |
| .P |
| 4. Do not end before extending characters or spacing marks or the "zero-width |
| joiner" character. Characters with the "mark" property always have the |
| "extend" grapheme breaking property. |
| .P |
| 5. Do not end after prepend characters. |
| .P |
| 6. Do not break within emoji modifier sequences or emoji zwj sequences. That |
| is, do not break between characters with the Extended_Pictographic property. |
| Extend and ZWJ characters are allowed between the characters. |
| .P |
| 7. Do not break within emoji flag sequences. That is, do not break between |
| regional indicator (RI) characters if there are an odd number of RI characters |
| before the break point. |
| .P |
| 8. Otherwise, end the cluster. |
| . |
| . |
| .\" HTML <a name="extraprops"></a> |
| .SS PCRE2's additional properties |
| .rs |
| .sp |
| As well as the standard Unicode properties described above, PCRE2 supports four |
| more that make it possible to convert traditional escape sequences such as \ew |
| and \es to use Unicode properties. PCRE2 uses these non-standard, non-Perl |
| properties internally when PCRE2_UCP is set. However, they may also be used |
| explicitly. These properties are: |
| .sp |
| Xan Any alphanumeric character |
| Xps Any POSIX space character |
| Xsp Any Perl space character |
| Xwd Any Perl "word" character |
| .sp |
| Xan matches characters that have either the L (letter) or the N (number) |
| property. Xps matches the characters tab, linefeed, vertical tab, form feed, or |
| carriage return, and any other character that has the Z (separator) property. |
| Xsp is the same as Xps; in PCRE1 it used to exclude vertical tab, for Perl |
| compatibility, but Perl changed. Xwd matches the same characters as Xan, plus |
| underscore. |
| .P |
| There is another non-standard property, Xuc, which matches any character that |
| can be represented by a Universal Character Name in C++ and other programming |
| languages. These are the characters $, @, ` (grave accent), and all characters |
| with Unicode code points greater than or equal to U+00A0, except for the |
| surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are |
| excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH |
| where H is a hexadecimal digit. Note that the Xuc property does not match these |
| sequences but the characters that they represent.) |
| . |
| . |
| .\" HTML <a name="resetmatchstart"></a> |
| .SS "Resetting the match start" |
| .rs |
| .sp |
| In normal use, the escape sequence \eK causes any previously matched characters |
| not to be included in the final matched sequence that is returned. For example, |
| the pattern: |
| .sp |
| foo\eKbar |
| .sp |
| matches "foobar", but reports that it has matched "bar". \eK does not interact |
| with anchoring in any way. The pattern: |
| .sp |
| ^foo\eKbar |
| .sp |
| matches only when the subject begins with "foobar" (in single line mode), |
| though it again reports the matched string as "bar". This feature is similar to |
| a lookbehind assertion |
| .\" HTML <a href="#lookbehind"> |
| .\" </a> |
| (described below). |
| .\" |
| However, in this case, the part of the subject before the real match does not |
| have to be of fixed length, as lookbehind assertions do. The use of \eK does |
| not interfere with the setting of |
| .\" HTML <a href="#subpattern"> |
| .\" </a> |
| captured substrings. |
| .\" |
| For example, when the pattern |
| .sp |
| (foo)\eKbar |
| .sp |
| matches "foobar", the first substring is still set to "foo". |
| .P |
| Perl documents that the use of \eK within assertions is "not well defined". In |
| PCRE2, \eK is acted upon when it occurs inside positive assertions, but is |
| ignored in negative assertions. Note that when a pattern such as (?=ab\eK) |
| matches, the reported start of the match can be greater than the end of the |
| match. Using \eK in a lookbehind assertion at the start of a pattern can also |
| lead to odd effects. For example, consider this pattern: |
| .sp |
| (?<=\eKfoo)bar |
| .sp |
| If the subject is "foobar", a call to \fBpcre2_match()\fP with a starting |
| offset of 3 succeeds and reports the matching string as "foobar", that is, the |
| start of the reported match is earlier than where the match started. |
| . |
| . |
| .\" HTML <a name="smallassertions"></a> |
| .SS "Simple assertions" |
| .rs |
| .sp |
| The final use of backslash is for certain simple assertions. An assertion |
| specifies a condition that has to be met at a particular point in a match, |
| without consuming any characters from the subject string. The use of |
| subpatterns for more complicated assertions is described |
| .\" HTML <a href="#bigassertions"> |
| .\" </a> |
| below. |
| .\" |
| The backslashed assertions are: |
| .sp |
| \eb matches at a word boundary |
| \eB matches when not at a word boundary |
| \eA matches at the start of the subject |
| \eZ matches at the end of the subject |
| also matches before a newline at the end of the subject |
| \ez matches only at the end of the subject |
| \eG matches at the first matching position in the subject |
| .sp |
| Inside a character class, \eb has a different meaning; it matches the backspace |
| character. If any other of these assertions appears in a character class, an |
| "invalid escape sequence" error is generated. |
| .P |
| A word boundary is a position in the subject string where the current character |
| and the previous character do not both match \ew or \eW (i.e. one matches |
| \ew and the other matches \eW), or the start or end of the string if the |
| first or last character matches \ew, respectively. In a UTF mode, the meanings |
| of \ew and \eW can be changed by setting the PCRE2_UCP option. When this is |
| done, it also affects \eb and \eB. Neither PCRE2 nor Perl has a separate "start |
| of word" or "end of word" metasequence. However, whatever follows \eb normally |
| determines which it is. For example, the fragment \eba matches "a" at the start |
| of a word. |
| .P |
| The \eA, \eZ, and \ez assertions differ from the traditional circumflex and |
| dollar (described in the next section) in that they only ever match at the very |
| start and end of the subject string, whatever options are set. Thus, they are |
| independent of multiline mode. These three assertions are not affected by the |
| PCRE2_NOTBOL or PCRE2_NOTEOL options, which affect only the behaviour of the |
| circumflex and dollar metacharacters. However, if the \fIstartoffset\fP |
| argument of \fBpcre2_match()\fP is non-zero, indicating that matching is to |
| start at a point other than the beginning of the subject, \eA can never match. |
| The difference between \eZ and \ez is that \eZ matches before a newline at the |
| end of the string as well as at the very end, whereas \ez matches only at the |
| end. |
| .P |
| The \eG assertion is true only when the current matching position is at the |
| start point of the matching process, as specified by the \fIstartoffset\fP |
| argument of \fBpcre2_match()\fP. It differs from \eA when the value of |
| \fIstartoffset\fP is non-zero. By calling \fBpcre2_match()\fP multiple times |
| with appropriate arguments, you can mimic Perl's /g option, and it is in this |
| kind of implementation where \eG can be useful. |
| .P |
| Note, however, that PCRE2's implementation of \eG, being true at the starting |
| character of the matching process, is subtly different from Perl's, which |
| defines it as true at the end of the previous match. In Perl, these can be |
| different when the previously matched string was empty. Because PCRE2 does just |
| one match at a time, it cannot reproduce this behaviour. |
| .P |
| If all the alternatives of a pattern begin with \eG, the expression is anchored |
| to the starting match position, and the "anchored" flag is set in the compiled |
| regular expression. |
| . |
| . |
| .SH "CIRCUMFLEX AND DOLLAR" |
| .rs |
| .sp |
| The circumflex and dollar metacharacters are zero-width assertions. That is, |
| they test for a particular condition being true without consuming any |
| characters from the subject string. These two metacharacters are concerned with |
| matching the starts and ends of lines. If the newline convention is set so that |
| only the two-character sequence CRLF is recognized as a newline, isolated CR |
| and LF characters are treated as ordinary data characters, and are not |
| recognized as newlines. |
| .P |
| Outside a character class, in the default matching mode, the circumflex |
| character is an assertion that is true only if the current matching point is at |
| the start of the subject string. If the \fIstartoffset\fP argument of |
| \fBpcre2_match()\fP is non-zero, or if PCRE2_NOTBOL is set, circumflex can |
| never match if the PCRE2_MULTILINE option is unset. Inside a character class, |
| circumflex has an entirely different meaning |
| .\" HTML <a href="#characterclass"> |
| .\" </a> |
| (see below). |
| .\" |
| .P |
| Circumflex need not be the first character of the pattern if a number of |
| alternatives are involved, but it should be the first thing in each alternative |
| in which it appears if the pattern is ever to match that branch. If all |
| possible alternatives start with a circumflex, that is, if the pattern is |
| constrained to match only at the start of the subject, it is said to be an |
| "anchored" pattern. (There are also other constructs that can cause a pattern |
| to be anchored.) |
| .P |
| The dollar character is an assertion that is true only if the current matching |
| point is at the end of the subject string, or immediately before a newline at |
| the end of the string (by default), unless PCRE2_NOTEOL is set. Note, however, |
| that it does not actually match the newline. Dollar need not be the last |
| character of the pattern if a number of alternatives are involved, but it |
| should be the last item in any branch in which it appears. Dollar has no |
| special meaning in a character class. |
| .P |
| The meaning of dollar can be changed so that it matches only at the very end of |
| the string, by setting the PCRE2_DOLLAR_ENDONLY option at compile time. This |
| does not affect the \eZ assertion. |
| .P |
| The meanings of the circumflex and dollar metacharacters are changed if the |
| PCRE2_MULTILINE option is set. When this is the case, a dollar character |
| matches before any newlines in the string, as well as at the very end, and a |
| circumflex matches immediately after internal newlines as well as at the start |
| of the subject string. It does not match after a newline that ends the string, |
| for compatibility with Perl. However, this can be changed by setting the |
| PCRE2_ALT_CIRCUMFLEX option. |
| .P |
| For example, the pattern /^abc$/ matches the subject string "def\enabc" (where |
| \en represents a newline) in multiline mode, but not otherwise. Consequently, |
| patterns that are anchored in single line mode because all branches start with |
| ^ are not anchored in multiline mode, and a match for circumflex is possible |
| when the \fIstartoffset\fP argument of \fBpcre2_match()\fP is non-zero. The |
| PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set. |
| .P |
| When the newline convention (see |
| .\" HTML <a href="#newlines"> |
| .\" </a> |
| "Newline conventions" |
| .\" |
| below) recognizes the two-character sequence CRLF as a newline, this is |
| preferred, even if the single characters CR and LF are also recognized as |
| newlines. For example, if the newline convention is "any", a multiline mode |
| circumflex matches before "xyz" in the string "abc\er\enxyz" rather than after |
| CR, even though CR on its own is a valid newline. (It also matches at the very |
| start of the string, of course.) |
| .P |
| Note that the sequences \eA, \eZ, and \ez can be used to match the start and |
| end of the subject in both modes, and if all branches of a pattern start with |
| \eA it is always anchored, whether or not PCRE2_MULTILINE is set. |
| . |
| . |
| .\" HTML <a name="fullstopdot"></a> |
| .SH "FULL STOP (PERIOD, DOT) AND \eN" |
| .rs |
| .sp |
| Outside a character class, a dot in the pattern matches any one character in |
| the subject string except (by default) a character that signifies the end of a |
| line. |
| .P |
| When a line ending is defined as a single character, dot never matches that |
| character; when the two-character sequence CRLF is used, dot does not match CR |
| if it is immediately followed by LF, but otherwise it matches all characters |
| (including isolated CRs and LFs). When any Unicode line endings are being |
| recognized, dot does not match CR or LF or any of the other line ending |
| characters. |
| .P |
| The behaviour of dot with regard to newlines can be changed. If the |
| PCRE2_DOTALL option is set, a dot matches any one character, without exception. |
| If the two-character sequence CRLF is present in the subject string, it takes |
| two dots to match it. |
| .P |
| The handling of dot is entirely independent of the handling of circumflex and |
| dollar, the only relationship being that they both involve newlines. Dot has no |
| special meaning in a character class. |
| .P |
| The escape sequence \eN when not followed by an opening brace behaves like a |
| dot, except that it is not affected by the PCRE2_DOTALL option. In other words, |
| it matches any character except one that signifies the end of a line. |
| .P |
| When \eN is followed by an opening brace it has a different meaning. See the |
| section entitled |
| .\" HTML <a href="digitsafterbackslash"> |
| .\" </a> |
| "Non-printing characters" |
| .\" |
| above for details. Perl also uses \eN{name} to specify characters by Unicode |
| name; PCRE2 does not support this. |
| . |
| . |
| .SH "MATCHING A SINGLE CODE UNIT" |
| .rs |
| .sp |
| Outside a character class, the escape sequence \eC matches any one code unit, |
| whether or not a UTF mode is set. In the 8-bit library, one code unit is one |
| byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is a |
| 32-bit unit. Unlike a dot, \eC always matches line-ending characters. The |
| feature is provided in Perl in order to match individual bytes in UTF-8 mode, |
| but it is unclear how it can usefully be used. |
| .P |
| Because \eC breaks up characters into individual code units, matching one unit |
| with \eC in UTF-8 or UTF-16 mode means that the rest of the string may start |
| with a malformed UTF character. This has undefined results, because PCRE2 |
| assumes that it is matching character by character in a valid UTF string (by |
| default it checks the subject string's validity at the start of processing |
| unless the PCRE2_NO_UTF_CHECK option is used). |
| .P |
| An application can lock out the use of \eC by setting the |
| PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also possible to |
| build PCRE2 with the use of \eC permanently disabled. |
| .P |
| PCRE2 does not allow \eC to appear in lookbehind assertions |
| .\" HTML <a href="#lookbehind"> |
| .\" </a> |
| (described below) |
| .\" |
| in UTF-8 or UTF-16 modes, because this would make it impossible to calculate |
| the length of the lookbehind. Neither the alternative matching function |
| \fBpcre2_dfa_match()\fP nor the JIT optimizer support \eC in these UTF modes. |
| The former gives a match-time error; the latter fails to optimize and so the |
| match is always run using the interpreter. |
| .P |
| In the 32-bit library, however, \eC is always supported (when not explicitly |
| locked out) because it always matches a single code unit, whether or not UTF-32 |
| is specified. |
| .P |
| In general, the \eC escape sequence is best avoided. However, one way of using |
| it that avoids the problem of malformed UTF-8 or UTF-16 characters is to use a |
| lookahead to check the length of the next character, as in this pattern, which |
| could be used with a UTF-8 string (ignore white space and line breaks): |
| .sp |
| (?| (?=[\ex00-\ex7f])(\eC) | |
| (?=[\ex80-\ex{7ff}])(\eC)(\eC) | |
| (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) | |
| (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC)) |
| .sp |
| In this example, a group that starts with (?| resets the capturing parentheses |
| numbers in each alternative (see |
| .\" HTML <a href="#dupsubpatternnumber"> |
| .\" </a> |
| "Duplicate Subpattern Numbers" |
| .\" |
| below). The assertions at the start of each branch check the next UTF-8 |
| character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The |
| character's individual bytes are then captured by the appropriate number of |
| \eC groups. |
| . |
| . |
| .\" HTML <a name="characterclass"></a> |
| .SH "SQUARE BRACKETS AND CHARACTER CLASSES" |
| .rs |
| .sp |
| An opening square bracket introduces a character class, terminated by a closing |
| square bracket. A closing square bracket on its own is not special by default. |
| If a closing square bracket is required as a member of the class, it should be |
| the first data character in the class (after an initial circumflex, if present) |
| or escaped with a backslash. This means that, by default, an empty class cannot |
| be defined. However, if the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing |
| square bracket at the start does end the (empty) class. |
| .P |
| A character class matches a single character in the subject. A matched |
| character must be in the set of characters defined by the class, unless the |
| first character in the class definition is a circumflex, in which case the |
| subject character must not be in the set defined by the class. If a circumflex |
| is actually required as a member of the class, ensure it is not the first |
| character, or escape it with a backslash. |
| .P |
| For example, the character class [aeiou] matches any lower case vowel, while |
| [^aeiou] matches any character that is not a lower case vowel. Note that a |
| circumflex is just a convenient notation for specifying the characters that |
| are in the class by enumerating those that are not. A class that starts with a |
| circumflex is not an assertion; it still consumes a character from the subject |
| string, and therefore it fails if the current pointer is at the end of the |
| string. |
| .P |
| Characters in a class may be specified by their code points using \eo, \ex, or |
| \eN{U+hh..} in the usual way. When caseless matching is set, any letters in a |
| class represent both their upper case and lower case versions, so for example, |
| a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not |
| match "A", whereas a caseful version would. |
| .P |
| Characters that might indicate line breaks are never treated in any special way |
| when matching character classes, whatever line-ending sequence is in use, and |
| whatever setting of the PCRE2_DOTALL and PCRE2_MULTILINE options is used. A |
| class such as [^a] always matches one of these characters. |
| .P |
| The generic character type escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, |
| \eS, \ev, \eV, \ew, and \eW may appear in a character class, and add the |
| characters that they match to the class. For example, [\edABCDEF] matches any |
| hexadecimal digit. In UTF modes, the PCRE2_UCP option affects the meanings of |
| \ed, \es, \ew and their upper case partners, just as it does when they appear |
| outside a character class, as described in the section entitled |
| .\" HTML <a href="#genericchartypes"> |
| .\" </a> |
| "Generic character types" |
| .\" |
| above. The escape sequence \eb has a different meaning inside a character |
| class; it matches the backspace character. The sequences \eB, \eR, and \eX are |
| not special inside a character class. Like any other unrecognized escape |
| sequences, they cause an error. The same is true for \eN when not followed by |
| an opening brace. |
| .P |
| The minus (hyphen) character can be used to specify a range of characters in a |
| character class. For example, [d-m] matches any letter between d and m, |
| inclusive. If a minus character is required in a class, it must be escaped with |
| a backslash or appear in a position where it cannot be interpreted as |
| indicating a range, typically as the first or last character in the class, |
| or immediately after a range. For example, [b-d-z] matches letters in the range |
| b to d, a hyphen character, or z. |
| .P |
| Perl treats a hyphen as a literal if it appears before or after a POSIX class |
| (see below) or before or after a character type escape such as as \ed or \eH. |
| However, unless the hyphen is the last character in the class, Perl outputs a |
| warning in its warning mode, as this is most likely a user error. As PCRE2 has |
| no facility for warning, an error is given in these cases. |
| .P |
| It is not possible to have the literal character "]" as the end character of a |
| range. A pattern such as [W-]46] is interpreted as a class of two characters |
| ("W" and "-") followed by a literal string "46]", so it would match "W46]" or |
| "-46]". However, if the "]" is escaped with a backslash it is interpreted as |
| the end of range, so [W-\e]46] is interpreted as a class containing a range |
| followed by two other characters. The octal or hexadecimal representation of |
| "]" can also be used to end a range. |
| .P |
| Ranges normally include all code points between the start and end characters, |
| inclusive. They can also be used for code points specified numerically, for |
| example [\e000-\e037]. Ranges can include any characters that are valid for the |
| current mode. In any UTF mode, the so-called "surrogate" characters (those |
| whose code points lie between 0xd800 and 0xdfff inclusive) may not be specified |
| explicitly by default (the PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables |
| this check). However, ranges such as [\ex{d7ff}-\ex{e000}], which include the |
| surrogates, are always permitted. |
| .P |
| There is a special case in EBCDIC environments for ranges whose end points are |
| both specified as literal letters in the same case. For compatibility with |
| Perl, EBCDIC code points within the range that are not letters are omitted. For |
| example, [h-k] matches only four characters, even though the codes for h and k |
| are 0x88 and 0x92, a range of 11 code points. However, if the range is |
| specified numerically, for example, [\ex88-\ex92] or [h-\ex92], all code points |
| are included. |
| .P |
| If a range that includes letters is used when caseless matching is set, it |
| matches the letters in either case. For example, [W-c] is equivalent to |
| [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character |
| tables for a French locale are in use, [\exc8-\excb] matches accented E |
| characters in both cases. |
| .P |
| A circumflex can conveniently be used with the upper case character types to |
| specify a more restricted set of characters than the matching lower case type. |
| For example, the class [^\eW_] matches any letter or digit, but not underscore, |
| whereas [\ew] includes underscore. A positive character class should be read as |
| "something OR something OR ..." and a negative class as "NOT something AND NOT |
| something AND NOT ...". |
| .P |
| The only metacharacters that are recognized in character classes are backslash, |
| hyphen (only where it can be interpreted as specifying a range), circumflex |
| (only at the start), opening square bracket (only when it can be interpreted as |
| introducing a POSIX class name, or for a special compatibility feature - see |
| the next two sections), and the terminating closing square bracket. However, |
| escaping other non-alphanumeric characters does no harm. |
| . |
| . |
| .SH "POSIX CHARACTER CLASSES" |
| .rs |
| .sp |
| Perl supports the POSIX notation for character classes. This uses names |
| enclosed by [: and :] within the enclosing square brackets. PCRE2 also supports |
| this notation. For example, |
| .sp |
| [01[:alpha:]%] |
| .sp |
| matches "0", "1", any alphabetic character, or "%". The supported class names |
| are: |
| .sp |
| alnum letters and digits |
| alpha letters |
| ascii character codes 0 - 127 |
| blank space or tab only |
| cntrl control characters |
| digit decimal digits (same as \ed) |
| graph printing characters, excluding space |
| lower lower case letters |
| print printing characters, including space |
| punct printing characters, excluding letters and digits and space |
| space white space (the same as \es from PCRE2 8.34) |
| upper upper case letters |
| word "word" characters (same as \ew) |
| xdigit hexadecimal digits |
| .sp |
| The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), |
| and space (32). If locale-specific matching is taking place, the list of space |
| characters may be different; there may be fewer or more of them. "Space" and |
| \es match the same set of characters. |
| .P |
| The name "word" is a Perl extension, and "blank" is a GNU extension from Perl |
| 5.8. Another Perl extension is negation, which is indicated by a ^ character |
| after the colon. For example, |
| .sp |
| [12[:^digit:]] |
| .sp |
| matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the POSIX |
| syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not |
| supported, and an error is given if they are encountered. |
| .P |
| By default, characters with values greater than 127 do not match any of the |
| POSIX character classes, although this may be different for characters in the |
| range 128-255 when locale-specific matching is happening. However, if the |
| PCRE2_UCP option is passed to \fBpcre2_compile()\fP, some of the classes are |
| changed so that Unicode character properties are used. This is achieved by |
| replacing certain POSIX classes with other sequences, as follows: |
| .sp |
| [:alnum:] becomes \ep{Xan} |
| [:alpha:] becomes \ep{L} |
| [:blank:] becomes \eh |
| [:cntrl:] becomes \ep{Cc} |
| [:digit:] becomes \ep{Nd} |
| [:lower:] becomes \ep{Ll} |
| [:space:] becomes \ep{Xps} |
| [:upper:] becomes \ep{Lu} |
| [:word:] becomes \ep{Xwd} |
| .sp |
| Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX |
| classes are handled specially in UCP mode: |
| .TP 10 |
| [:graph:] |
| This matches characters that have glyphs that mark the page when printed. In |
| Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf |
| properties, except for: |
| .sp |
| U+061C Arabic Letter Mark |
| U+180E Mongolian Vowel Separator |
| U+2066 - U+2069 Various "isolate"s |
| .sp |
| .TP 10 |
| [:print:] |
| This matches the same characters as [:graph:] plus space characters that are |
| not controls, that is, characters with the Zs property. |
| .TP 10 |
| [:punct:] |
| This matches all characters that have the Unicode P (punctuation) property, |
| plus those characters with code points less than 256 that have the S (Symbol) |
| property. |
| .P |
| The other POSIX classes are unchanged, and match only characters with code |
| points less than 256. |
| . |
| . |
| .SH "COMPATIBILITY FEATURE FOR WORD BOUNDARIES" |
| .rs |
| .sp |
| In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly |
| syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of |
| word". PCRE2 treats these items as follows: |
| .sp |
| [[:<:]] is converted to \eb(?=\ew) |
| [[:>:]] is converted to \eb(?<=\ew) |
| .sp |
| Only these exact character sequences are recognized. A sequence such as |
| [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is |
| not compatible with Perl. It is provided to help migrations from other |
| environments, and is best not used in any new patterns. Note that \eb matches |
| at the start and the end of a word (see |
| .\" HTML <a href="#smallassertions"> |
| .\" </a> |
| "Simple assertions" |
| .\" |
| above), and in a Perl-style pattern the preceding or following character |
| normally shows which is wanted, without the need for the assertions that are |
| used above in order to give exactly the POSIX behaviour. |
| . |
| . |
| .SH "VERTICAL BAR" |
| .rs |
| .sp |
| Vertical bar characters are used to separate alternative patterns. For example, |
| the pattern |
| .sp |
| gilbert|sullivan |
| .sp |
| matches either "gilbert" or "sullivan". Any number of alternatives may appear, |
| and an empty alternative is permitted (matching the empty string). The matching |
| process tries each alternative in turn, from left to right, and the first one |
| that succeeds is used. If the alternatives are within a subpattern |
| .\" HTML <a href="#subpattern"> |
| .\" </a> |
| (defined below), |
| .\" |
| "succeeds" means matching the rest of the main pattern as well as the |
| alternative in the subpattern. |
| . |
| . |
| .SH "INTERNAL OPTION SETTING" |
| .rs |
| .sp |
| The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, |
| PCRE2_EXTENDED, PCRE2_EXTENDED_MORE, and PCRE2_NO_AUTO_CAPTURE options can be |
| changed from within the pattern by a sequence of letters enclosed between "(?" |
| and ")". These options are Perl-compatible, and are described in detail in the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| documentation. The option letters are: |
| .sp |
| i for PCRE2_CASELESS |
| m for PCRE2_MULTILINE |
| n for PCRE2_NO_AUTO_CAPTURE |
| s for PCRE2_DOTALL |
| x for PCRE2_EXTENDED |
| xx for PCRE2_EXTENDED_MORE |
| .sp |
| For example, (?im) sets caseless, multiline matching. It is also possible to |
| unset these options by preceding the relevant letters with a hyphen, for |
| example (?-im). The two "extended" options are not independent; unsetting either |
| one cancels the effects of both of them. |
| .P |
| A combined setting and unsetting such as (?im-sx), which sets PCRE2_CASELESS |
| and PCRE2_MULTILINE while unsetting PCRE2_DOTALL and PCRE2_EXTENDED, is also |
| permitted. Only one hyphen may appear in the options string. If a letter |
| appears both before and after the hyphen, the option is unset. An empty options |
| setting "(?)" is allowed. Needless to say, it has no effect. |
| .P |
| If the first character following (? is a circumflex, it causes all of the above |
| options to be unset. Thus, (?^) is equivalent to (?-imnsx). Letters may follow |
| the circumflex to cause some options to be re-instated, but a hyphen may not |
| appear. |
| .P |
| The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be changed in |
| the same way as the Perl-compatible options by using the characters J and U |
| respectively. However, these are not unset by (?^). |
| .P |
| When one of these option changes occurs at top level (that is, not inside |
| subpattern parentheses), the change applies to the remainder of the pattern |
| that follows. An option change within a subpattern (see below for a description |
| of subpatterns) affects only that part of the subpattern that follows it, so |
| .sp |
| (a(?i)b)c |
| .sp |
| matches abc and aBc and no other strings (assuming PCRE2_CASELESS is not used). |
| By this means, options can be made to have different settings in different |
| parts of the pattern. Any changes made in one alternative do carry on |
| into subsequent branches within the same subpattern. For example, |
| .sp |
| (a(?i)b|c) |
| .sp |
| matches "ab", "aB", "c", and "C", even though when matching "C" the first |
| branch is abandoned before the option setting. This is because the effects of |
| option settings happen at compile time. There would be some very weird |
| behaviour otherwise. |
| .P |
| As a convenient shorthand, if any option settings are required at the start of |
| a non-capturing subpattern (see the next section), the option letters may |
| appear between the "?" and the ":". Thus the two patterns |
| .sp |
| (?i:saturday|sunday) |
| (?:(?i)saturday|sunday) |
| .sp |
| match exactly the same set of strings. |
| .P |
| \fBNote:\fP There are other PCRE2-specific options that can be set by the |
| application when the compiling function is called. The pattern can contain |
| special leading sequences such as (*CRLF) to override what the application has |
| set or what has been defaulted. Details are given in the section entitled |
| .\" HTML <a href="#newlineseq"> |
| .\" </a> |
| "Newline sequences" |
| .\" |
| above. There are also the (*UTF) and (*UCP) leading sequences that can be used |
| to set UTF and Unicode property modes; they are equivalent to setting the |
| PCRE2_UTF and PCRE2_UCP options, respectively. However, the application can set |
| the PCRE2_NEVER_UTF and PCRE2_NEVER_UCP options, which lock out the use of the |
| (*UTF) and (*UCP) sequences. |
| . |
| . |
| .\" HTML <a name="subpattern"></a> |
| .SH SUBPATTERNS |
| .rs |
| .sp |
| Subpatterns are delimited by parentheses (round brackets), which can be nested. |
| Turning part of a pattern into a subpattern does two things: |
| .sp |
| 1. It localizes a set of alternatives. For example, the pattern |
| .sp |
| cat(aract|erpillar|) |
| .sp |
| matches "cataract", "caterpillar", or "cat". Without the parentheses, it would |
| match "cataract", "erpillar" or an empty string. |
| .sp |
| 2. It sets up the subpattern as a capturing subpattern. This means that, when |
| the whole pattern matches, the portion of the subject string that matched the |
| subpattern is passed back to the caller, separately from the portion that |
| matched the whole pattern. (This applies only to the traditional matching |
| function; the DFA matching function does not support capturing.) |
| .P |
| Opening parentheses are counted from left to right (starting from 1) to obtain |
| numbers for the capturing subpatterns. For example, if the string "the red |
| king" is matched against the pattern |
| .sp |
| the ((red|white) (king|queen)) |
| .sp |
| the captured substrings are "red king", "red", and "king", and are numbered 1, |
| 2, and 3, respectively. |
| .P |
| The fact that plain parentheses fulfil two functions is not always helpful. |
| There are often times when a grouping subpattern is required without a |
| capturing requirement. If an opening parenthesis is followed by a question mark |
| and a colon, the subpattern does not do any capturing, and is not counted when |
| computing the number of any subsequent capturing subpatterns. For example, if |
| the string "the white queen" is matched against the pattern |
| .sp |
| the ((?:red|white) (king|queen)) |
| .sp |
| the captured substrings are "white queen" and "queen", and are numbered 1 and |
| 2. The maximum number of capturing subpatterns is 65535. |
| .P |
| As a convenient shorthand, if any option settings are required at the start of |
| a non-capturing subpattern, the option letters may appear between the "?" and |
| the ":". Thus the two patterns |
| .sp |
| (?i:saturday|sunday) |
| (?:(?i)saturday|sunday) |
| .sp |
| match exactly the same set of strings. Because alternative branches are tried |
| from left to right, and options are not reset until the end of the subpattern |
| is reached, an option setting in one branch does affect subsequent branches, so |
| the above patterns match "SUNDAY" as well as "Saturday". |
| . |
| . |
| .\" HTML <a name="dupsubpatternnumber"></a> |
| .SH "DUPLICATE SUBPATTERN NUMBERS" |
| .rs |
| .sp |
| Perl 5.10 introduced a feature whereby each alternative in a subpattern uses |
| the same numbers for its capturing parentheses. Such a subpattern starts with |
| (?| and is itself a non-capturing subpattern. For example, consider this |
| pattern: |
| .sp |
| (?|(Sat)ur|(Sun))day |
| .sp |
| Because the two alternatives are inside a (?| group, both sets of capturing |
| parentheses are numbered one. Thus, when the pattern matches, you can look |
| at captured substring number one, whichever alternative matched. This construct |
| is useful when you want to capture part, but not all, of one of a number of |
| alternatives. Inside a (?| group, parentheses are numbered as usual, but the |
| number is reset at the start of each branch. The numbers of any capturing |
| parentheses that follow the subpattern start after the highest number used in |
| any branch. The following example is taken from the Perl documentation. The |
| numbers underneath show in which buffer the captured content will be stored. |
| .sp |
| # before ---------------branch-reset----------- after |
| / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x |
| # 1 2 2 3 2 3 4 |
| .sp |
| A backreference to a numbered subpattern uses the most recent value that is |
| set for that number by any subpattern. The following pattern matches "abcabc" |
| or "defdef": |
| .sp |
| /(?|(abc)|(def))\e1/ |
| .sp |
| In contrast, a subroutine call to a numbered subpattern always refers to the |
| first one in the pattern with the given number. The following pattern matches |
| "abcabc" or "defabc": |
| .sp |
| /(?|(abc)|(def))(?1)/ |
| .sp |
| A relative reference such as (?-1) is no different: it is just a convenient way |
| of computing an absolute group number. |
| .P |
| If a |
| .\" HTML <a href="#conditions"> |
| .\" </a> |
| condition test |
| .\" |
| for a subpattern's having matched refers to a non-unique number, the test is |
| true if any of the subpatterns of that number have matched. |
| .P |
| An alternative approach to using this "branch reset" feature is to use |
| duplicate named subpatterns, as described in the next section. |
| . |
| . |
| .SH "NAMED SUBPATTERNS" |
| .rs |
| .sp |
| Identifying capturing parentheses by number is simple, but it can be very hard |
| to keep track of the numbers in complicated patterns. Furthermore, if an |
| expression is modified, the numbers may change. To help with this difficulty, |
| PCRE2 supports the naming of capturing subpatterns. This feature was not added |
| to Perl until release 5.10. Python had the feature earlier, and PCRE1 |
| introduced it at release 4.0, using the Python syntax. PCRE2 supports both the |
| Perl and the Python syntax. |
| .P |
| In PCRE2, a capturing subpattern can be named in one of three ways: |
| (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. Names |
| consist of up to 32 alphanumeric characters and underscores, but must start |
| with a non-digit. References to capturing parentheses from other parts of the |
| pattern, such as |
| .\" HTML <a href="#backreferences"> |
| .\" </a> |
| backreferences, |
| .\" |
| .\" HTML <a href="#recursion"> |
| .\" </a> |
| recursion, |
| .\" |
| and |
| .\" HTML <a href="#conditions"> |
| .\" </a> |
| conditions, |
| .\" |
| can all be made by name as well as by number. |
| .P |
| Named capturing parentheses are allocated numbers as well as names, exactly as |
| if the names were not present. In both PCRE2 and Perl, capturing subpatterns |
| are primarily identified by numbers; any names are just aliases for these |
| numbers. The PCRE2 API provides function calls for extracting the complete |
| name-to-number translation table from a compiled pattern, as well as |
| convenience functions for extracting captured substrings by name. |
| .P |
| \fBWarning:\fP When more than one subpattern has the same number, as described |
| in the previous section, a name given to one of them applies to all of them. |
| Perl allows identically numbered subpatterns to have different names. Consider |
| this pattern, where there are two capturing subpatterns, both numbered 1: |
| .sp |
| (?|(?<AA>aa)|(?<BB>bb)) |
| .sp |
| Perl allows this, with both names AA and BB as aliases of group 1. Thus, after |
| a successful match, both names yield the same value (either "aa" or "bb"). |
| .P |
| In an attempt to reduce confusion, PCRE2 does not allow the same group number |
| to be associated with more than one name. The example above provokes a |
| compile-time error. However, there is still scope for confusion. Consider this |
| pattern: |
| .sp |
| (?|(?<AA>aa)|(bb)) |
| .sp |
| Although the second subpattern number 1 is not explicitly named, the name AA is |
| still an alias for subpattern 1. Whether the pattern matches "aa" or "bb", a |
| reference by name to group AA yields the matched string. |
| .P |
| By default, a name must be unique within a pattern, except that duplicate names |
| are permitted for subpatterns with the same number, for example: |
| .sp |
| (?|(?<AA>aa)|(?<AA>bb)) |
| .sp |
| The duplicate name constraint can be disabled by setting the PCRE2_DUPNAMES |
| option at compile time, or by the use of (?J) within the pattern. Duplicate |
| names can be useful for patterns where only one instance of the named |
| parentheses can match. Suppose you want to match the name of a weekday, either |
| as a 3-letter abbreviation or as the full name, and in both cases you want to |
| extract the abbreviation. This pattern (ignoring the line breaks) does the job: |
| .sp |
| (?<DN>Mon|Fri|Sun)(?:day)?| |
| (?<DN>Tue)(?:sday)?| |
| (?<DN>Wed)(?:nesday)?| |
| (?<DN>Thu)(?:rsday)?| |
| (?<DN>Sat)(?:urday)? |
| .sp |
| There are five capturing substrings, but only one is ever set after a match. |
| The convenience functions for extracting the data by name returns the substring |
| for the first (and in this example, the only) subpattern of that name that |
| matched. This saves searching to find which numbered subpattern it was. (An |
| alternative way of solving this problem is to use a "branch reset" subpattern, |
| as described in the previous section.) |
| .P |
| If you make a backreference to a non-unique named subpattern from elsewhere in |
| the pattern, the subpatterns to which the name refers are checked in the order |
| in which they appear in the overall pattern. The first one that is set is used |
| for the reference. For example, this pattern matches both "foofoo" and |
| "barbar" but not "foobar" or "barfoo": |
| .sp |
| (?:(?<n>foo)|(?<n>bar))\ek<n> |
| .sp |
| .P |
| If you make a subroutine call to a non-unique named subpattern, the one that |
| corresponds to the first occurrence of the name is used. In the absence of |
| duplicate numbers this is the one with the lowest number. |
| .P |
| If you use a named reference in a condition |
| test (see the |
| .\" |
| .\" HTML <a href="#conditions"> |
| .\" </a> |
| section about conditions |
| .\" |
| below), either to check whether a subpattern has matched, or to check for |
| recursion, all subpatterns with the same name are tested. If the condition is |
| true for any one of them, the overall condition is true. This is the same |
| behaviour as testing by number. For further details of the interfaces for |
| handling named subpatterns, see the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| documentation. |
| . |
| . |
| .SH REPETITION |
| .rs |
| .sp |
| Repetition is specified by quantifiers, which can follow any of the following |
| items: |
| .sp |
| a literal data character |
| the dot metacharacter |
| the \eC escape sequence |
| the \eX escape sequence |
| the \eR escape sequence |
| an escape such as \ed or \epL that matches a single character |
| a character class |
| a backreference |
| a parenthesized subpattern (including most assertions) |
| a subroutine call to a subpattern (recursive or otherwise) |
| .sp |
| The general repetition quantifier specifies a minimum and maximum number of |
| permitted matches, by giving the two numbers in curly brackets (braces), |
| separated by a comma. The numbers must be less than 65536, and the first must |
| be less than or equal to the second. For example: |
| .sp |
| z{2,4} |
| .sp |
| matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special |
| character. If the second number is omitted, but the comma is present, there is |
| no upper limit; if the second number and the comma are both omitted, the |
| quantifier specifies an exact number of required matches. Thus |
| .sp |
| [aeiou]{3,} |
| .sp |
| matches at least 3 successive vowels, but may match many more, whereas |
| .sp |
| \ed{8} |
| .sp |
| matches exactly 8 digits. An opening curly bracket that appears in a position |
| where a quantifier is not allowed, or one that does not match the syntax of a |
| quantifier, is taken as a literal character. For example, {,6} is not a |
| quantifier, but a literal string of four characters. |
| .P |
| In UTF modes, quantifiers apply to characters rather than to individual code |
| units. Thus, for example, \ex{100}{2} matches two characters, each of |
| which is represented by a two-byte sequence in a UTF-8 string. Similarly, |
| \eX{3} matches three Unicode extended grapheme clusters, each of which may be |
| several code units long (and they may be of different lengths). |
| .P |
| The quantifier {0} is permitted, causing the expression to behave as if the |
| previous item and the quantifier were not present. This may be useful for |
| subpatterns that are referenced as |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| subroutines |
| .\" |
| from elsewhere in the pattern (but see also the section entitled |
| .\" HTML <a href="#subdefine"> |
| .\" </a> |
| "Defining subpatterns for use by reference only" |
| .\" |
| below). Items other than subpatterns that have a {0} quantifier are omitted |
| from the compiled pattern. |
| .P |
| For convenience, the three most common quantifiers have single-character |
| abbreviations: |
| .sp |
| * is equivalent to {0,} |
| + is equivalent to {1,} |
| ? is equivalent to {0,1} |
| .sp |
| It is possible to construct infinite loops by following a subpattern that can |
| match no characters with a quantifier that has no upper limit, for example: |
| .sp |
| (a?)* |
| .sp |
| Earlier versions of Perl and PCRE1 used to give an error at compile time for |
| such patterns. However, because there are cases where this can be useful, such |
| patterns are now accepted, but if any repetition of the subpattern does in fact |
| match no characters, the loop is forcibly broken. |
| .P |
| By default, the quantifiers are "greedy", that is, they match as much as |
| possible (up to the maximum number of permitted times), without causing the |
| rest of the pattern to fail. The classic example of where this gives problems |
| is in trying to match comments in C programs. These appear between /* and */ |
| and within the comment, individual * and / characters may appear. An attempt to |
| match C comments by applying the pattern |
| .sp |
| /\e*.*\e*/ |
| .sp |
| to the string |
| .sp |
| /* first comment */ not comment /* second comment */ |
| .sp |
| fails, because it matches the entire string owing to the greediness of the .* |
| item. |
| .P |
| If a quantifier is followed by a question mark, it ceases to be greedy, and |
| instead matches the minimum number of times possible, so the pattern |
| .sp |
| /\e*.*?\e*/ |
| .sp |
| does the right thing with the C comments. The meaning of the various |
| quantifiers is not otherwise changed, just the preferred number of matches. |
| Do not confuse this use of question mark with its use as a quantifier in its |
| own right. Because it has two uses, it can sometimes appear doubled, as in |
| .sp |
| \ed??\ed |
| .sp |
| which matches one digit by preference, but can match two if that is the only |
| way the rest of the pattern matches. |
| .P |
| If the PCRE2_UNGREEDY option is set (an option that is not available in Perl), |
| the quantifiers are not greedy by default, but individual ones can be made |
| greedy by following them with a question mark. In other words, it inverts the |
| default behaviour. |
| .P |
| When a parenthesized subpattern is quantified with a minimum repeat count that |
| is greater than 1 or with a limited maximum, more memory is required for the |
| compiled pattern, in proportion to the size of the minimum or maximum. |
| .P |
| If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option (equivalent |
| to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is |
| implicitly anchored, because whatever follows will be tried against every |
| character position in the subject string, so there is no point in retrying the |
| overall match at any position after the first. PCRE2 normally treats such a |
| pattern as though it were preceded by \eA. |
| .P |
| In cases where it is known that the subject string contains no newlines, it is |
| worth setting PCRE2_DOTALL in order to obtain this optimization, or |
| alternatively, using ^ to indicate anchoring explicitly. |
| .P |
| However, there are some cases where the optimization cannot be used. When .* |
| is inside capturing parentheses that are the subject of a backreference |
| elsewhere in the pattern, a match at the start may fail where a later one |
| succeeds. Consider, for example: |
| .sp |
| (.*)abc\e1 |
| .sp |
| If the subject is "xyz123abc123" the match point is the fourth character. For |
| this reason, such a pattern is not implicitly anchored. |
| .P |
| Another case where implicit anchoring is not applied is when the leading .* is |
| inside an atomic group. Once again, a match at the start may fail where a later |
| one succeeds. Consider this pattern: |
| .sp |
| (?>.*?a)b |
| .sp |
| It matches "ab" in the subject "aab". The use of the backtracking control verbs |
| (*PRUNE) and (*SKIP) also disable this optimization, and there is an option, |
| PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly. |
| .P |
| When a capturing subpattern is repeated, the value captured is the substring |
| that matched the final iteration. For example, after |
| .sp |
| (tweedle[dume]{3}\es*)+ |
| .sp |
| has matched "tweedledum tweedledee" the value of the captured substring is |
| "tweedledee". However, if there are nested capturing subpatterns, the |
| corresponding captured values may have been set in previous iterations. For |
| example, after |
| .sp |
| (a|(b))+ |
| .sp |
| matches "aba" the value of the second captured substring is "b". |
| . |
| . |
| .\" HTML <a name="atomicgroup"></a> |
| .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS" |
| .rs |
| .sp |
| With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") |
| repetition, failure of what follows normally causes the repeated item to be |
| re-evaluated to see if a different number of repeats allows the rest of the |
| pattern to match. Sometimes it is useful to prevent this, either to change the |
| nature of the match, or to cause it fail earlier than it otherwise might, when |
| the author of the pattern knows there is no point in carrying on. |
| .P |
| Consider, for example, the pattern \ed+foo when applied to the subject line |
| .sp |
| 123456bar |
| .sp |
| After matching all 6 digits and then failing to match "foo", the normal |
| action of the matcher is to try again with only 5 digits matching the \ed+ |
| item, and then with 4, and so on, before ultimately failing. "Atomic grouping" |
| (a term taken from Jeffrey Friedl's book) provides the means for specifying |
| that once a subpattern has matched, it is not to be re-evaluated in this way. |
| .P |
| If we use atomic grouping for the previous example, the matcher gives up |
| immediately on failing to match "foo" the first time. The notation is a kind of |
| special parenthesis, starting with (?> as in this example: |
| .sp |
| (?>\ed+)foo |
| .sp |
| This kind of parenthesis "locks up" the part of the pattern it contains once |
| it has matched, and a failure further into the pattern is prevented from |
| backtracking into it. Backtracking past it to previous items, however, works as |
| normal. |
| .P |
| An alternative description is that a subpattern of this type matches exactly |
| the string of characters that an identical standalone pattern would match, if |
| anchored at the current point in the subject string. |
| .P |
| Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as |
| the above example can be thought of as a maximizing repeat that must swallow |
| everything it can. So, while both \ed+ and \ed+? are prepared to adjust the |
| number of digits they match in order to make the rest of the pattern match, |
| (?>\ed+) can only match an entire sequence of digits. |
| .P |
| Atomic groups in general can of course contain arbitrarily complicated |
| subpatterns, and can be nested. However, when the subpattern for an atomic |
| group is just a single repeated item, as in the example above, a simpler |
| notation, called a "possessive quantifier" can be used. This consists of an |
| additional + character following a quantifier. Using this notation, the |
| previous example can be rewritten as |
| .sp |
| \ed++foo |
| .sp |
| Note that a possessive quantifier can be used with an entire group, for |
| example: |
| .sp |
| (abc|xyz){2,3}+ |
| .sp |
| Possessive quantifiers are always greedy; the setting of the PCRE2_UNGREEDY |
| option is ignored. They are a convenient notation for the simpler forms of |
| atomic group. However, there is no difference in the meaning of a possessive |
| quantifier and the equivalent atomic group, though there may be a performance |
| difference; possessive quantifiers should be slightly faster. |
| .P |
| The possessive quantifier syntax is an extension to the Perl 5.8 syntax. |
| Jeffrey Friedl originated the idea (and the name) in the first edition of his |
| book. Mike McCloskey liked it, so implemented it when he built Sun's Java |
| package, and PCRE1 copied it from there. It ultimately found its way into Perl |
| at release 5.10. |
| .P |
| PCRE2 has an optimization that automatically "possessifies" certain simple |
| pattern constructs. For example, the sequence A+B is treated as A++B because |
| there is no point in backtracking into a sequence of A's when B must follow. |
| This feature can be disabled by the PCRE2_NO_AUTOPOSSESS option, or starting |
| the pattern with (*NO_AUTO_POSSESS). |
| .P |
| When a pattern contains an unlimited repeat inside a subpattern that can itself |
| be repeated an unlimited number of times, the use of an atomic group is the |
| only way to avoid some failing matches taking a very long time indeed. The |
| pattern |
| .sp |
| (\eD+|<\ed+>)*[!?] |
| .sp |
| matches an unlimited number of substrings that either consist of non-digits, or |
| digits enclosed in <>, followed by either ! or ?. When it matches, it runs |
| quickly. However, if it is applied to |
| .sp |
| aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa |
| .sp |
| it takes a long time before reporting failure. This is because the string can |
| be divided between the internal \eD+ repeat and the external * repeat in a |
| large number of ways, and all have to be tried. (The example uses [!?] rather |
| than a single character at the end, because both PCRE2 and Perl have an |
| optimization that allows for fast failure when a single character is used. They |
| remember the last single character that is required for a match, and fail early |
| if it is not present in the string.) If the pattern is changed so that it uses |
| an atomic group, like this: |
| .sp |
| ((?>\eD+)|<\ed+>)*[!?] |
| .sp |
| sequences of non-digits cannot be broken, and failure happens quickly. |
| . |
| . |
| .\" HTML <a name="backreferences"></a> |
| .SH "BACKREFERENCES" |
| .rs |
| .sp |
| Outside a character class, a backslash followed by a digit greater than 0 (and |
| possibly further digits) is a backreference to a capturing subpattern earlier |
| (that is, to its left) in the pattern, provided there have been that many |
| previous capturing left parentheses. |
| .P |
| However, if the decimal number following the backslash is less than 8, it is |
| always taken as a backreference, and causes an error only if there are not |
| that many capturing left parentheses in the entire pattern. In other words, the |
| parentheses that are referenced need not be to the left of the reference for |
| numbers less than 8. A "forward backreference" of this type can make sense |
| when a repetition is involved and the subpattern to the right has participated |
| in an earlier iteration. |
| .P |
| It is not possible to have a numerical "forward backreference" to a subpattern |
| whose number is 8 or more using this syntax because a sequence such as \e50 is |
| interpreted as a character defined in octal. See the subsection entitled |
| "Non-printing characters" |
| .\" HTML <a href="#digitsafterbackslash"> |
| .\" </a> |
| above |
| .\" |
| for further details of the handling of digits following a backslash. There is |
| no such problem when named parentheses are used. A backreference to any |
| subpattern is possible using named parentheses (see below). |
| .P |
| Another way of avoiding the ambiguity inherent in the use of digits following a |
| backslash is to use the \eg escape sequence. This escape must be followed by a |
| signed or unsigned number, optionally enclosed in braces. These examples are |
| all identical: |
| .sp |
| (ring), \e1 |
| (ring), \eg1 |
| (ring), \eg{1} |
| .sp |
| An unsigned number specifies an absolute reference without the ambiguity that |
| is present in the older syntax. It is also useful when literal digits follow |
| the reference. A signed number is a relative reference. Consider this example: |
| .sp |
| (abc(def)ghi)\eg{-1} |
| .sp |
| The sequence \eg{-1} is a reference to the most recently started capturing |
| subpattern before \eg, that is, is it equivalent to \e2 in this example. |
| Similarly, \eg{-2} would be equivalent to \e1. The use of relative references |
| can be helpful in long patterns, and also in patterns that are created by |
| joining together fragments that contain references within themselves. |
| .P |
| The sequence \eg{+1} is a reference to the next capturing subpattern. This kind |
| of forward reference can be useful it patterns that repeat. Perl does not |
| support the use of + in this way. |
| .P |
| A backreference matches whatever actually matched the capturing subpattern in |
| the current subject string, rather than anything matching the subpattern |
| itself (see |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| "Subpatterns as subroutines" |
| .\" |
| below for a way of doing that). So the pattern |
| .sp |
| (sens|respons)e and \e1ibility |
| .sp |
| matches "sense and sensibility" and "response and responsibility", but not |
| "sense and responsibility". If caseful matching is in force at the time of the |
| backreference, the case of letters is relevant. For example, |
| .sp |
| ((?i)rah)\es+\e1 |
| .sp |
| matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original |
| capturing subpattern is matched caselessly. |
| .P |
| There are several different ways of writing backreferences to named |
| subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or |
| \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified |
| backreference syntax, in which \eg can be used for both numeric and named |
| references, is also supported. We could rewrite the above example in any of |
| the following ways: |
| .sp |
| (?<p1>(?i)rah)\es+\ek<p1> |
| (?'p1'(?i)rah)\es+\ek{p1} |
| (?P<p1>(?i)rah)\es+(?P=p1) |
| (?<p1>(?i)rah)\es+\eg{p1} |
| .sp |
| A subpattern that is referenced by name may appear in the pattern before or |
| after the reference. |
| .P |
| There may be more than one backreference to the same subpattern. If a |
| subpattern has not actually been used in a particular match, any backreferences |
| to it always fail by default. For example, the pattern |
| .sp |
| (a|(bc))\e2 |
| .sp |
| always fails if it starts to match "a" rather than "bc". However, if the |
| PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backreference to an |
| unset value matches an empty string. |
| .P |
| Because there may be many capturing parentheses in a pattern, all digits |
| following a backslash are taken as part of a potential backreference number. |
| If the pattern continues with a digit character, some delimiter must be used to |
| terminate the backreference. If the PCRE2_EXTENDED or PCRE2_EXTENDED_MORE |
| option is set, this can be white space. Otherwise, the \eg{ syntax or an empty |
| comment (see |
| .\" HTML <a href="#comments"> |
| .\" </a> |
| "Comments" |
| .\" |
| below) can be used. |
| . |
| . |
| .SS "Recursive backreferences" |
| .rs |
| .sp |
| A backreference that occurs inside the parentheses to which it refers fails |
| when the subpattern is first used, so, for example, (a\e1) never matches. |
| However, such references can be useful inside repeated subpatterns. For |
| example, the pattern |
| .sp |
| (a|b\e1)+ |
| .sp |
| matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of |
| the subpattern, the backreference matches the character string corresponding |
| to the previous iteration. In order for this to work, the pattern must be such |
| that the first iteration does not need to match the backreference. This can be |
| done using alternation, as in the example above, or by a quantifier with a |
| minimum of zero. |
| .P |
| Backreferences of this type cause the group that they reference to be treated |
| as an |
| .\" HTML <a href="#atomicgroup"> |
| .\" </a> |
| atomic group. |
| .\" |
| Once the whole group has been matched, a subsequent matching failure cannot |
| cause backtracking into the middle of the group. |
| . |
| . |
| .\" HTML <a name="bigassertions"></a> |
| .SH ASSERTIONS |
| .rs |
| .sp |
| An assertion is a test on the characters following or preceding the current |
| matching point that does not consume any characters. The simple assertions |
| coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described |
| .\" HTML <a href="#smallassertions"> |
| .\" </a> |
| above. |
| .\" |
| .P |
| More complicated assertions are coded as subpatterns. There are two kinds: |
| those that look ahead of the current position in the subject string, and those |
| that look behind it, and in each case an assertion may be positive (must |
| succeed for matching to continue) or negative (must not succeed for matching to |
| continue). An assertion subpattern is matched in the normal way, except that, |
| when matching continues after a successful assertion, the matching position in |
| the subject string is as it was before the assertion was processed. |
| .P |
| Assertion subpatterns are not capturing subpatterns. If an assertion contains |
| capturing subpatterns within it, these are counted for the purposes of |
| numbering the capturing subpatterns in the whole pattern. Within each branch of |
| an assertion, locally captured substrings may be referenced in the usual way. |
| For example, a sequence such as (.)\eg{-1} can be used to check that two |
| adjacent characters are the same. |
| .P |
| When a branch within an assertion fails to match, any substrings that were |
| captured are discarded (as happens with any pattern branch that fails to |
| match). A negative assertion succeeds only when all its branches fail to match; |
| this means that no captured substrings are ever retained after a successful |
| negative assertion. When an assertion contains a matching branch, what happens |
| depends on the type of assertion. |
| .P |
| For a positive assertion, internally captured substrings in the successful |
| branch are retained, and matching continues with the next pattern item after |
| the assertion. For a negative assertion, a matching branch means that the |
| assertion has failed. If the assertion is being used as a condition in a |
| .\" HTML <a href="#conditions"> |
| .\" </a> |
| conditional subpattern |
| .\" |
| (see below), captured substrings are retained, because matching continues with |
| the "no" branch of the condition. For other failing negative assertions, |
| control passes to the previous backtracking point, thus discarding any captured |
| strings within the assertion. |
| .P |
| For compatibility with Perl, most assertion subpatterns may be repeated; though |
| it makes no sense to assert the same thing several times, the side effect of |
| capturing parentheses may occasionally be useful. However, an assertion that |
| forms the condition for a conditional subpattern may not be quantified. In |
| practice, for other assertions, there only three cases: |
| .sp |
| (1) If the quantifier is {0}, the assertion is never obeyed during matching. |
| However, it may contain internal capturing parenthesized groups that are called |
| from elsewhere via the |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| subroutine mechanism. |
| .\" |
| .sp |
| (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it |
| were {0,1}. At run time, the rest of the pattern match is tried with and |
| without the assertion, the order depending on the greediness of the quantifier. |
| .sp |
| (3) If the minimum repetition is greater than zero, the quantifier is ignored. |
| The assertion is obeyed just once when encountered during matching. |
| . |
| . |
| .SS "Lookahead assertions" |
| .rs |
| .sp |
| Lookahead assertions start with (?= for positive assertions and (?! for |
| negative assertions. For example, |
| .sp |
| \ew+(?=;) |
| .sp |
| matches a word followed by a semicolon, but does not include the semicolon in |
| the match, and |
| .sp |
| foo(?!bar) |
| .sp |
| matches any occurrence of "foo" that is not followed by "bar". Note that the |
| apparently similar pattern |
| .sp |
| (?!foo)bar |
| .sp |
| does not find an occurrence of "bar" that is preceded by something other than |
| "foo"; it finds any occurrence of "bar" whatsoever, because the assertion |
| (?!foo) is always true when the next three characters are "bar". A |
| lookbehind assertion is needed to achieve the other effect. |
| .P |
| If you want to force a matching failure at some point in a pattern, the most |
| convenient way to do it is with (?!) because an empty string always matches, so |
| an assertion that requires there not to be an empty string must always fail. |
| The backtracking control verb (*FAIL) or (*F) is a synonym for (?!). |
| . |
| . |
| .\" HTML <a name="lookbehind"></a> |
| .SS "Lookbehind assertions" |
| .rs |
| .sp |
| Lookbehind assertions start with (?<= for positive assertions and (?<! for |
| negative assertions. For example, |
| .sp |
| (?<!foo)bar |
| .sp |
| does find an occurrence of "bar" that is not preceded by "foo". The contents of |
| a lookbehind assertion are restricted such that all the strings it matches must |
| have a fixed length. However, if there are several top-level alternatives, they |
| do not all have to have the same fixed length. Thus |
| .sp |
| (?<=bullock|donkey) |
| .sp |
| is permitted, but |
| .sp |
| (?<!dogs?|cats?) |
| .sp |
| causes an error at compile time. Branches that match different length strings |
| are permitted only at the top level of a lookbehind assertion. This is an |
| extension compared with Perl, which requires all branches to match the same |
| length of string. An assertion such as |
| .sp |
| (?<=ab(c|de)) |
| .sp |
| is not permitted, because its single top-level branch can match two different |
| lengths, but it is acceptable to PCRE2 if rewritten to use two top-level |
| branches: |
| .sp |
| (?<=abc|abde) |
| .sp |
| In some cases, the escape sequence \eK |
| .\" HTML <a href="#resetmatchstart"> |
| .\" </a> |
| (see above) |
| .\" |
| can be used instead of a lookbehind assertion to get round the fixed-length |
| restriction. |
| .P |
| The implementation of lookbehind assertions is, for each alternative, to |
| temporarily move the current position back by the fixed length and then try to |
| match. If there are insufficient characters before the current position, the |
| assertion fails. |
| .P |
| In UTF-8 and UTF-16 modes, PCRE2 does not allow the \eC escape (which matches a |
| single code unit even in a UTF mode) to appear in lookbehind assertions, |
| because it makes it impossible to calculate the length of the lookbehind. The |
| \eX and \eR escapes, which can match different numbers of code units, are never |
| permitted in lookbehinds. |
| .P |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| "Subroutine" |
| .\" |
| calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long |
| as the subpattern matches a fixed-length string. However, |
| .\" HTML <a href="#recursion"> |
| .\" </a> |
| recursion, |
| .\" |
| that is, a "subroutine" call into a group that is already active, |
| is not supported. |
| .P |
| Perl does not support backreferences in lookbehinds. PCRE2 does support them, |
| but only if certain conditions are met. The PCRE2_MATCH_UNSET_BACKREF option |
| must not be set, there must be no use of (?| in the pattern (it creates |
| duplicate subpattern numbers), and if the backreference is by name, the name |
| must be unique. Of course, the referenced subpattern must itself be of fixed |
| length. The following pattern matches words containing at least two characters |
| that begin and end with the same character: |
| .sp |
| \eb(\ew)\ew++(?<=\e1) |
| .P |
| Possessive quantifiers can be used in conjunction with lookbehind assertions to |
| specify efficient matching of fixed-length strings at the end of subject |
| strings. Consider a simple pattern such as |
| .sp |
| abcd$ |
| .sp |
| when applied to a long string that does not match. Because matching proceeds |
| from left to right, PCRE2 will look for each "a" in the subject and then see if |
| what follows matches the rest of the pattern. If the pattern is specified as |
| .sp |
| ^.*abcd$ |
| .sp |
| the initial .* matches the entire string at first, but when this fails (because |
| there is no following "a"), it backtracks to match all but the last character, |
| then all but the last two characters, and so on. Once again the search for "a" |
| covers the entire string, from right to left, so we are no better off. However, |
| if the pattern is written as |
| .sp |
| ^.*+(?<=abcd) |
| .sp |
| there can be no backtracking for the .*+ item because of the possessive |
| quantifier; it can match only the entire string. The subsequent lookbehind |
| assertion does a single test on the last four characters. If it fails, the |
| match fails immediately. For long strings, this approach makes a significant |
| difference to the processing time. |
| . |
| . |
| .SS "Using multiple assertions" |
| .rs |
| .sp |
| Several assertions (of any sort) may occur in succession. For example, |
| .sp |
| (?<=\ed{3})(?<!999)foo |
| .sp |
| matches "foo" preceded by three digits that are not "999". Notice that each of |
| the assertions is applied independently at the same point in the subject |
| string. First there is a check that the previous three characters are all |
| digits, and then there is a check that the same three characters are not "999". |
| This pattern does \fInot\fP match "foo" preceded by six characters, the first |
| of which are digits and the last three of which are not "999". For example, it |
| doesn't match "123abcfoo". A pattern to do that is |
| .sp |
| (?<=\ed{3}...)(?<!999)foo |
| .sp |
| This time the first assertion looks at the preceding six characters, checking |
| that the first three are digits, and then the second assertion checks that the |
| preceding three characters are not "999". |
| .P |
| Assertions can be nested in any combination. For example, |
| .sp |
| (?<=(?<!foo)bar)baz |
| .sp |
| matches an occurrence of "baz" that is preceded by "bar" which in turn is not |
| preceded by "foo", while |
| .sp |
| (?<=\ed{3}(?!999)...)foo |
| .sp |
| is another pattern that matches "foo" preceded by three digits and any three |
| characters that are not "999". |
| . |
| . |
| .\" HTML <a name="conditions"></a> |
| .SH "CONDITIONAL SUBPATTERNS" |
| .rs |
| .sp |
| It is possible to cause the matching process to obey a subpattern |
| conditionally or to choose between two alternative subpatterns, depending on |
| the result of an assertion, or whether a specific capturing subpattern has |
| already been matched. The two possible forms of conditional subpattern are: |
| .sp |
| (?(condition)yes-pattern) |
| (?(condition)yes-pattern|no-pattern) |
| .sp |
| If the condition is satisfied, the yes-pattern is used; otherwise the |
| no-pattern (if present) is used. An absent no-pattern is equivalent to an empty |
| string (it always matches). If there are more than two alternatives in the |
| subpattern, a compile-time error occurs. Each of the two alternatives may |
| itself contain nested subpatterns of any form, including conditional |
| subpatterns; the restriction to two alternatives applies only at the level of |
| the condition. This pattern fragment is an example where the alternatives are |
| complex: |
| .sp |
| (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) |
| .sp |
| .P |
| There are five kinds of condition: references to subpatterns, references to |
| recursion, two pseudo-conditions called DEFINE and VERSION, and assertions. |
| . |
| . |
| .SS "Checking for a used subpattern by number" |
| .rs |
| .sp |
| If the text between the parentheses consists of a sequence of digits, the |
| condition is true if a capturing subpattern of that number has previously |
| matched. If there is more than one capturing subpattern with the same number |
| (see the earlier |
| .\" |
| .\" HTML <a href="#recursion"> |
| .\" </a> |
| section about duplicate subpattern numbers), |
| .\" |
| the condition is true if any of them have matched. An alternative notation is |
| to precede the digits with a plus or minus sign. In this case, the subpattern |
| number is relative rather than absolute. The most recently opened parentheses |
| can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside |
| loops it can also make sense to refer to subsequent groups. The next |
| parentheses to be opened can be referenced as (?(+1), and so on. (The value |
| zero in any of these forms is not used; it provokes a compile-time error.) |
| .P |
| Consider the following pattern, which contains non-significant white space to |
| make it more readable (assume the PCRE2_EXTENDED option) and to divide it into |
| three parts for ease of discussion: |
| .sp |
| ( \e( )? [^()]+ (?(1) \e) ) |
| .sp |
| The first part matches an optional opening parenthesis, and if that |
| character is present, sets it as the first captured substring. The second part |
| matches one or more characters that are not parentheses. The third part is a |
| conditional subpattern that tests whether or not the first set of parentheses |
| matched. If they did, that is, if subject started with an opening parenthesis, |
| the condition is true, and so the yes-pattern is executed and a closing |
| parenthesis is required. Otherwise, since no-pattern is not present, the |
| subpattern matches nothing. In other words, this pattern matches a sequence of |
| non-parentheses, optionally enclosed in parentheses. |
| .P |
| If you were embedding this pattern in a larger one, you could use a relative |
| reference: |
| .sp |
| ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ... |
| .sp |
| This makes the fragment independent of the parentheses in the larger pattern. |
| . |
| . |
| .SS "Checking for a used subpattern by name" |
| .rs |
| .sp |
| Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used |
| subpattern by name. For compatibility with earlier versions of PCRE1, which had |
| this facility before Perl, the syntax (?(name)...) is also recognized. Note, |
| however, that undelimited names consisting of the letter R followed by digits |
| are ambiguous (see the following section). |
| .P |
| Rewriting the above example to use a named subpattern gives this: |
| .sp |
| (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) ) |
| .sp |
| If the name used in a condition of this kind is a duplicate, the test is |
| applied to all subpatterns of the same name, and is true if any one of them has |
| matched. |
| . |
| . |
| .SS "Checking for pattern recursion" |
| .rs |
| .sp |
| "Recursion" in this sense refers to any subroutine-like call from one part of |
| the pattern to another, whether or not it is actually recursive. See the |
| sections entitled |
| .\" HTML <a href="#recursion"> |
| .\" </a> |
| "Recursive patterns" |
| .\" |
| and |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| "Subpatterns as subroutines" |
| .\" |
| below for details of recursion and subpattern calls. |
| .P |
| If a condition is the string (R), and there is no subpattern with the name R, |
| the condition is true if matching is currently in a recursion or subroutine |
| call to the whole pattern or any subpattern. If digits follow the letter R, and |
| there is no subpattern with that name, the condition is true if the most recent |
| call is into a subpattern with the given number, which must exist somewhere in |
| the overall pattern. This is a contrived example that is equivalent to a+b: |
| .sp |
| ((?(R1)a+|(?1)b)) |
| .sp |
| However, in both cases, if there is a subpattern with a matching name, the |
| condition tests for its being set, as described in the section above, instead |
| of testing for recursion. For example, creating a group with the name R1 by |
| adding (?<R1>) to the above pattern completely changes its meaning. |
| .P |
| If a name preceded by ampersand follows the letter R, for example: |
| .sp |
| (?(R&name)...) |
| .sp |
| the condition is true if the most recent recursion is into a subpattern of that |
| name (which must exist within the pattern). |
| .P |
| This condition does not check the entire recursion stack. It tests only the |
| current level. If the name used in a condition of this kind is a duplicate, the |
| test is applied to all subpatterns of the same name, and is true if any one of |
| them is the most recent recursion. |
| .P |
| At "top level", all these recursion test conditions are false. |
| . |
| . |
| .\" HTML <a name="subdefine"></a> |
| .SS "Defining subpatterns for use by reference only" |
| .rs |
| .sp |
| If the condition is the string (DEFINE), the condition is always false, even if |
| there is a group with the name DEFINE. In this case, there may be only one |
| alternative in the subpattern. It is always skipped if control reaches this |
| point in the pattern; the idea of DEFINE is that it can be used to define |
| subroutines that can be referenced from elsewhere. (The use of |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| subroutines |
| .\" |
| is described below.) For example, a pattern to match an IPv4 address such as |
| "192.168.23.245" could be written like this (ignore white space and line |
| breaks): |
| .sp |
| (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) ) |
| \eb (?&byte) (\e.(?&byte)){3} \eb |
| .sp |
| The first part of the pattern is a DEFINE group inside which a another group |
| named "byte" is defined. This matches an individual component of an IPv4 |
| address (a number less than 256). When matching takes place, this part of the |
| pattern is skipped because DEFINE acts like a false condition. The rest of the |
| pattern uses references to the named group to match the four dot-separated |
| components of an IPv4 address, insisting on a word boundary at each end. |
| . |
| . |
| .SS "Checking the PCRE2 version" |
| .rs |
| .sp |
| Programs that link with a PCRE2 library can check the version by calling |
| \fBpcre2_config()\fP with appropriate arguments. Users of applications that do |
| not have access to the underlying code cannot do this. A special "condition" |
| called VERSION exists to allow such users to discover which version of PCRE2 |
| they are dealing with by using this condition to match a string such as |
| "yesno". VERSION must be followed either by "=" or ">=" and a version number. |
| For example: |
| .sp |
| (?(VERSION>=10.4)yes|no) |
| .sp |
| This pattern matches "yes" if the PCRE2 version is greater or equal to 10.4, or |
| "no" otherwise. The fractional part of the version number may not contain more |
| than two digits. |
| . |
| . |
| .SS "Assertion conditions" |
| .rs |
| .sp |
| If the condition is not in any of the above formats, it must be an assertion. |
| This may be a positive or negative lookahead or lookbehind assertion. Consider |
| this pattern, again containing non-significant white space, and with the two |
| alternatives on the second line: |
| .sp |
| (?(?=[^a-z]*[a-z]) |
| \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} ) |
| .sp |
| The condition is a positive lookahead assertion that matches an optional |
| sequence of non-letters followed by a letter. In other words, it tests for the |
| presence of at least one letter in the subject. If a letter is found, the |
| subject is matched against the first alternative; otherwise it is matched |
| against the second. This pattern matches strings in one of the two forms |
| dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits. |
| .P |
| When an assertion that is a condition contains capturing subpatterns, any |
| capturing that occurs in a matching branch is retained afterwards, for both |
| positive and negative assertions, because matching always continues after the |
| assertion, whether it succeeds or fails. (Compare non-conditional assertions, |
| when captures are retained only for positive assertions that succeed.) |
| . |
| . |
| .\" HTML <a name="comments"></a> |
| .SH COMMENTS |
| .rs |
| .sp |
| There are two ways of including comments in patterns that are processed by |
| PCRE2. In both cases, the start of the comment must not be in a character |
| class, nor in the middle of any other sequence of related characters such as |
| (?: or a subpattern name or number. The characters that make up a comment play |
| no part in the pattern matching. |
| .P |
| The sequence (?# marks the start of a comment that continues up to the next |
| closing parenthesis. Nested parentheses are not permitted. If the |
| PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, an unescaped # character |
| also introduces a comment, which in this case continues to immediately after |
| the next newline character or character sequence in the pattern. Which |
| characters are interpreted as newlines is controlled by an option passed to the |
| compiling function or by a special sequence at the start of the pattern, as |
| described in the section entitled |
| .\" HTML <a href="#newlines"> |
| .\" </a> |
| "Newline conventions" |
| .\" |
| above. Note that the end of this type of comment is a literal newline sequence |
| in the pattern; escape sequences that happen to represent a newline do not |
| count. For example, consider this pattern when PCRE2_EXTENDED is set, and the |
| default newline convention (a single linefeed character) is in force: |
| .sp |
| abc #comment \en still comment |
| .sp |
| On encountering the # character, \fBpcre2_compile()\fP skips along, looking for |
| a newline in the pattern. The sequence \en is still literal at this stage, so |
| it does not terminate the comment. Only an actual character with the code value |
| 0x0a (the default newline) does so. |
| . |
| . |
| .\" HTML <a name="recursion"></a> |
| .SH "RECURSIVE PATTERNS" |
| .rs |
| .sp |
| Consider the problem of matching a string in parentheses, allowing for |
| unlimited nested parentheses. Without the use of recursion, the best that can |
| be done is to use a pattern that matches up to some fixed depth of nesting. It |
| is not possible to handle an arbitrary nesting depth. |
| .P |
| For some time, Perl has provided a facility that allows regular expressions to |
| recurse (amongst other things). It does this by interpolating Perl code in the |
| expression at run time, and the code can refer to the expression itself. A Perl |
| pattern using code interpolation to solve the parentheses problem can be |
| created like this: |
| .sp |
| $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x; |
| .sp |
| The (?p{...}) item interpolates Perl code at run time, and in this case refers |
| recursively to the pattern in which it appears. |
| .P |
| Obviously, PCRE2 cannot support the interpolation of Perl code. Instead, it |
| supports special syntax for recursion of the entire pattern, and also for |
| individual subpattern recursion. After its introduction in PCRE1 and Python, |
| this kind of recursion was subsequently introduced into Perl at release 5.10. |
| .P |
| A special item that consists of (? followed by a number greater than zero and a |
| closing parenthesis is a recursive subroutine call of the subpattern of the |
| given number, provided that it occurs inside that subpattern. (If not, it is a |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| non-recursive subroutine |
| .\" |
| call, which is described in the next section.) The special item (?R) or (?0) is |
| a recursive call of the entire regular expression. |
| .P |
| This PCRE2 pattern solves the nested parentheses problem (assume the |
| PCRE2_EXTENDED option is set so that white space is ignored): |
| .sp |
| \e( ( [^()]++ | (?R) )* \e) |
| .sp |
| First it matches an opening parenthesis. Then it matches any number of |
| substrings which can either be a sequence of non-parentheses, or a recursive |
| match of the pattern itself (that is, a correctly parenthesized substring). |
| Finally there is a closing parenthesis. Note the use of a possessive quantifier |
| to avoid backtracking into sequences of non-parentheses. |
| .P |
| If this were part of a larger pattern, you would not want to recurse the entire |
| pattern, so instead you could use this: |
| .sp |
| ( \e( ( [^()]++ | (?1) )* \e) ) |
| .sp |
| We have put the pattern into parentheses, and caused the recursion to refer to |
| them instead of the whole pattern. |
| .P |
| In a larger pattern, keeping track of parenthesis numbers can be tricky. This |
| is made easier by the use of relative references. Instead of (?1) in the |
| pattern above you can write (?-2) to refer to the second most recently opened |
| parentheses preceding the recursion. In other words, a negative number counts |
| capturing parentheses leftwards from the point at which it is encountered. |
| .P |
| Be aware however, that if |
| .\" HTML <a href="#dupsubpatternnumber"> |
| .\" </a> |
| duplicate subpattern numbers |
| .\" |
| are in use, relative references refer to the earliest subpattern with the |
| appropriate number. Consider, for example: |
| .sp |
| (?|(a)|(b)) (c) (?-2) |
| .sp |
| The first two capturing groups (a) and (b) are both numbered 1, and group (c) |
| is number 2. When the reference (?-2) is encountered, the second most recently |
| opened parentheses has the number 1, but it is the first such group (the (a) |
| group) to which the recursion refers. This would be the same if an absolute |
| reference (?1) was used. In other words, relative references are just a |
| shorthand for computing a group number. |
| .P |
| It is also possible to refer to subsequently opened parentheses, by writing |
| references such as (?+2). However, these cannot be recursive because the |
| reference is not inside the parentheses that are referenced. They are always |
| .\" HTML <a href="#subpatternsassubroutines"> |
| .\" </a> |
| non-recursive subroutine |
| .\" |
| calls, as described in the next section. |
| .P |
| An alternative approach is to use named parentheses. The Perl syntax for this |
| is (?&name); PCRE1's earlier syntax (?P>name) is also supported. We could |
| rewrite the above example as follows: |
| .sp |
| (?<pn> \e( ( [^()]++ | (?&pn) )* \e) ) |
| .sp |
| If there is more than one subpattern with the same name, the earliest one is |
| used. |
| .P |
| The example pattern that we have been looking at contains nested unlimited |
| repeats, and so the use of a possessive quantifier for matching strings of |
| non-parentheses is important when applying the pattern to strings that do not |
| match. For example, when this pattern is applied to |
| .sp |
| (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() |
| .sp |
| it yields "no match" quickly. However, if a possessive quantifier is not used, |
| the match runs for a very long time indeed because there are so many different |
| ways the + and * repeats can carve up the subject, and all have to be tested |
| before failure can be reported. |
| .P |
| At the end of a match, the values of capturing parentheses are those from |
| the outermost level. If you want to obtain intermediate values, a callout |
| function can be used (see below and the |
| .\" HREF |
| \fBpcre2callout\fP |
| .\" |
| documentation). If the pattern above is matched against |
| .sp |
| (ab(cd)ef) |
| .sp |
| the value for the inner capturing parentheses (numbered 2) is "ef", which is |
| the last value taken on at the top level. If a capturing subpattern is not |
| matched at the top level, its final captured value is unset, even if it was |
| (temporarily) set at a deeper level during the matching process. |
| .P |
| Do not confuse the (?R) item with the condition (R), which tests for recursion. |
| Consider this pattern, which matches text in angle brackets, allowing for |
| arbitrary nesting. Only digits are allowed in nested brackets (that is, when |
| recursing), whereas any characters are permitted at the outer level. |
| .sp |
| < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * > |
| .sp |
| In this pattern, (?(R) is the start of a conditional subpattern, with two |
| different alternatives for the recursive and non-recursive cases. The (?R) item |
| is the actual recursive call. |
| . |
| . |
| .\" HTML <a name="recursiondifference"></a> |
| .SS "Differences in recursion processing between PCRE2 and Perl" |
| .rs |
| .sp |
| Some former differences between PCRE2 and Perl no longer exist. |
| .P |
| Before release 10.30, recursion processing in PCRE2 differed from Perl in that |
| a recursive subpattern call was always treated as an atomic group. That is, |
| once it had matched some of the subject string, it was never re-entered, even |
| if it contained untried alternatives and there was a subsequent matching |
| failure. (Historical note: PCRE implemented recursion before Perl did.) |
| .P |
| Starting with release 10.30, recursive subroutine calls are no longer treated |
| as atomic. That is, they can be re-entered to try unused alternatives if there |
| is a matching failure later in the pattern. This is now compatible with the way |
| Perl works. If you want a subroutine call to be atomic, you must explicitly |
| enclose it in an atomic group. |
| .P |
| Supporting backtracking into recursions simplifies certain types of recursive |
| pattern. For example, this pattern matches palindromic strings: |
| .sp |
| ^((.)(?1)\e2|.?)$ |
| .sp |
| The second branch in the group matches a single central character in the |
| palindrome when there are an odd number of characters, or nothing when there |
| are an even number of characters, but in order to work it has to be able to try |
| the second case when the rest of the pattern match fails. If you want to match |
| typical palindromic phrases, the pattern has to ignore all non-word characters, |
| which can be done like this: |
| .sp |
| ^\eW*+((.)\eW*+(?1)\eW*+\e2|\eW*+.?)\eW*+$ |
| .sp |
| If run with the PCRE2_CASELESS option, this pattern matches phrases such as "A |
| man, a plan, a canal: Panama!". Note the use of the possessive quantifier *+ to |
| avoid backtracking into sequences of non-word characters. Without this, PCRE2 |
| takes a great deal longer (ten times or more) to match typical phrases, and |
| Perl takes so long that you think it has gone into a loop. |
| .P |
| Another way in which PCRE2 and Perl used to differ in their recursion |
| processing is in the handling of captured values. Formerly in Perl, when a |
| subpattern was called recursively or as a subpattern (see the next section), it |
| had no access to any values that were captured outside the recursion, whereas |
| in PCRE2 these values can be referenced. Consider this pattern: |
| .sp |
| ^(.)(\e1|a(?2)) |
| .sp |
| This pattern matches "bab". The first capturing parentheses match "b", then in |
| the second group, when the backreference \e1 fails to match "b", the second |
| alternative matches "a" and then recurses. In the recursion, \e1 does now match |
| "b" and so the whole match succeeds. This match used to fail in Perl, but in |
| later versions (I tried 5.024) it now works. |
| . |
| . |
| .\" HTML <a name="subpatternsassubroutines"></a> |
| .SH "SUBPATTERNS AS SUBROUTINES" |
| .rs |
| .sp |
| If the syntax for a recursive subpattern call (either by number or by |
| name) is used outside the parentheses to which it refers, it operates a bit |
| like a subroutine in a programming language. More accurately, PCRE2 treats the |
| referenced subpattern as an independent subpattern which it tries to match at |
| the current matching position. The called subpattern may be defined before or |
| after the reference. A numbered reference can be absolute or relative, as in |
| these examples: |
| .sp |
| (...(absolute)...)...(?2)... |
| (...(relative)...)...(?-1)... |
| (...(?+1)...(relative)... |
| .sp |
| An earlier example pointed out that the pattern |
| .sp |
| (sens|respons)e and \e1ibility |
| .sp |
| matches "sense and sensibility" and "response and responsibility", but not |
| "sense and responsibility". If instead the pattern |
| .sp |
| (sens|respons)e and (?1)ibility |
| .sp |
| is used, it does match "sense and responsibility" as well as the other two |
| strings. Another example is given in the discussion of DEFINE above. |
| .P |
| Like recursions, subroutine calls used to be treated as atomic, but this |
| changed at PCRE2 release 10.30, so backtracking into subroutine calls can now |
| occur. However, any capturing parentheses that are set during the subroutine |
| call revert to their previous values afterwards. |
| .P |
| Processing options such as case-independence are fixed when a subpattern is |
| defined, so if it is used as a subroutine, such options cannot be changed for |
| different calls. For example, consider this pattern: |
| .sp |
| (abc)(?i:(?-1)) |
| .sp |
| It matches "abcabc". It does not match "abcABC" because the change of |
| processing option does not affect the called subpattern. |
| .P |
| The behaviour of |
| .\" HTML <a href="#backtrackcontrol"> |
| .\" </a> |
| backtracking control verbs |
| .\" |
| in subpatterns when called as subroutines is described in the section entitled |
| .\" HTML <a href="#btsub"> |
| .\" </a> |
| "Backtracking verbs in subroutines" |
| .\" |
| below. |
| . |
| . |
| .\" HTML <a name="onigurumasubroutines"></a> |
| .SH "ONIGURUMA SUBROUTINE SYNTAX" |
| .rs |
| .sp |
| For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or |
| a number enclosed either in angle brackets or single quotes, is an alternative |
| syntax for referencing a subpattern as a subroutine, possibly recursively. Here |
| are two of the examples used above, rewritten using this syntax: |
| .sp |
| (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) ) |
| (sens|respons)e and \eg'1'ibility |
| .sp |
| PCRE2 supports an extension to Oniguruma: if a number is preceded by a |
| plus or a minus sign it is taken as a relative reference. For example: |
| .sp |
| (abc)(?i:\eg<-1>) |
| .sp |
| Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP |
| synonymous. The former is a backreference; the latter is a subroutine call. |
| . |
| . |
| .SH CALLOUTS |
| .rs |
| .sp |
| Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl |
| code to be obeyed in the middle of matching a regular expression. This makes it |
| possible, amongst other things, to extract different substrings that match the |
| same pair of parentheses when there is a repetition. |
| .P |
| PCRE2 provides a similar feature, but of course it cannot obey arbitrary Perl |
| code. The feature is called "callout". The caller of PCRE2 provides an external |
| function by putting its entry point in a match context using the function |
| \fBpcre2_set_callout()\fP, and then passing that context to \fBpcre2_match()\fP |
| or \fBpcre2_dfa_match()\fP. If no match context is passed, or if the callout |
| entry point is set to NULL, callouts are disabled. |
| .P |
| Within a regular expression, (?C<arg>) indicates a point at which the external |
| function is to be called. There are two kinds of callout: those with a |
| numerical argument and those with a string argument. (?C) on its own with no |
| argument is treated as (?C0). A numerical argument allows the application to |
| distinguish between different callouts. String arguments were added for release |
| 10.20 to make it possible for script languages that use PCRE2 to embed short |
| scripts within patterns in a similar way to Perl. |
| .P |
| During matching, when PCRE2 reaches a callout point, the external function is |
| called. It is provided with the number or string argument of the callout, the |
| position in the pattern, and one item of data that is also set in the match |
| block. The callout function may cause matching to proceed, to backtrack, or to |
| fail. |
| .P |
| By default, PCRE2 implements a number of optimizations at matching time, and |
| one side-effect is that sometimes callouts are skipped. If you need all |
| possible callouts to happen, you need to set options that disable the relevant |
| optimizations. More details, including a complete description of the |
| programming interface to the callout function, are given in the |
| .\" HREF |
| \fBpcre2callout\fP |
| .\" |
| documentation. |
| . |
| . |
| .SS "Callouts with numerical arguments" |
| .rs |
| .sp |
| If you just want to have a means of identifying different callout points, put a |
| number less than 256 after the letter C. For example, this pattern has two |
| callout points: |
| .sp |
| (?C1)abc(?C2)def |
| .sp |
| If the PCRE2_AUTO_CALLOUT flag is passed to \fBpcre2_compile()\fP, numerical |
| callouts are automatically installed before each item in the pattern. They are |
| all numbered 255. If there is a conditional group in the pattern whose |
| condition is an assertion, an additional callout is inserted just before the |
| condition. An explicit callout may also be set at this position, as in this |
| example: |
| .sp |
| (?(?C9)(?=a)abc|def) |
| .sp |
| Note that this applies only to assertion conditions, not to other types of |
| condition. |
| . |
| . |
| .SS "Callouts with string arguments" |
| .rs |
| .sp |
| A delimited string may be used instead of a number as a callout argument. The |
| starting delimiter must be one of ` ' " ^ % # $ { and the ending delimiter is |
| the same as the start, except for {, where the ending delimiter is }. If the |
| ending delimiter is needed within the string, it must be doubled. For |
| example: |
| .sp |
| (?C'ab ''c'' d')xyz(?C{any text})pqr |
| .sp |
| The doubling is removed before the string is passed to the callout function. |
| . |
| . |
| .\" HTML <a name="backtrackcontrol"></a> |
| .SH "BACKTRACKING CONTROL" |
| .rs |
| .sp |
| There are a number of special "Backtracking Control Verbs" (to use Perl's |
| terminology) that modify the behaviour of backtracking during matching. They |
| are generally of the form (*VERB) or (*VERB:NAME). Some verbs take either form, |
| possibly behaving differently depending on whether or not a name is present. |
| .P |
| By default, for compatibility with Perl, a name is any sequence of characters |
| that does not include a closing parenthesis. The name is not processed in |
| any way, and it is not possible to include a closing parenthesis in the name. |
| This can be changed by setting the PCRE2_ALT_VERBNAMES option, but the result |
| is no longer Perl-compatible. |
| .P |
| When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to verb names |
| and only an unescaped closing parenthesis terminates the name. However, the |
| only backslash items that are permitted are \eQ, \eE, and sequences such as |
| \ex{100} that define character code points. Character type escapes such as \ed |
| are faulted. |
| .P |
| A closing parenthesis can be included in a name either as \e) or between \eQ |
| and \eE. In addition to backslash processing, if the PCRE2_EXTENDED or |
| PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb names is |
| skipped, and #-comments are recognized, exactly as in the rest of the pattern. |
| PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect verb names unless |
| PCRE2_ALT_VERBNAMES is also set. |
| .P |
| The maximum length of a name is 255 in the 8-bit library and 65535 in the |
| 16-bit and 32-bit libraries. If the name is empty, that is, if the closing |
| parenthesis immediately follows the colon, the effect is as if the colon were |
| not there. Any number of these verbs may occur in a pattern. |
| .P |
| Since these verbs are specifically related to backtracking, most of them can be |
| used only when the pattern is to be matched using the traditional matching |
| function, because that uses a backtracking algorithm. With the exception of |
| (*FAIL), which behaves like a failing negative assertion, the backtracking |
| control verbs cause an error if encountered by the DFA matching function. |
| .P |
| The behaviour of these verbs in |
| .\" HTML <a href="#btrepeat"> |
| .\" </a> |
| repeated groups, |
| .\" |
| .\" HTML <a href="#btassert"> |
| .\" </a> |
| assertions, |
| .\" |
| and in |
| .\" HTML <a href="#btsub"> |
| .\" </a> |
| subpatterns called as subroutines |
| .\" |
| (whether or not recursively) is documented below. |
| . |
| . |
| .\" HTML <a name="nooptimize"></a> |
| .SS "Optimizations that affect backtracking verbs" |
| .rs |
| .sp |
| PCRE2 contains some optimizations that are used to speed up matching by running |
| some checks at the start of each match attempt. For example, it may know the |
| minimum length of matching subject, or that a particular character must be |
| present. When one of these optimizations bypasses the running of a match, any |
| included backtracking verbs will not, of course, be processed. You can suppress |
| the start-of-match optimizations by setting the PCRE2_NO_START_OPTIMIZE option |
| when calling \fBpcre2_compile()\fP, or by starting the pattern with |
| (*NO_START_OPT). There is more discussion of this option in the section |
| entitled |
| .\" HTML <a href="pcre2api.html#compiling"> |
| .\" </a> |
| "Compiling a pattern" |
| .\" |
| in the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| documentation. |
| .P |
| Experiments with Perl suggest that it too has similar optimizations, and like |
| PCRE2, turning them off can change the result of a match. |
| . |
| . |
| .SS "Verbs that act immediately" |
| .rs |
| .sp |
| The following verbs act as soon as they are encountered. |
| .sp |
| (*ACCEPT) or (*ACCEPT:NAME) |
| .sp |
| This verb causes the match to end successfully, skipping the remainder of the |
| pattern. However, when it is inside a subpattern that is called as a |
| subroutine, only that subpattern is ended successfully. Matching then continues |
| at the outer level. If (*ACCEPT) in triggered in a positive assertion, the |
| assertion succeeds; in a negative assertion, the assertion fails. |
| .P |
| If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For |
| example: |
| .sp |
| A((?:A|B(*ACCEPT)|C)D) |
| .sp |
| This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by |
| the outer parentheses. |
| .sp |
| (*FAIL) or (*FAIL:NAME) |
| .sp |
| This verb causes a matching failure, forcing backtracking to occur. It may be |
| abbreviated to (*F). It is equivalent to (?!) but easier to read. The Perl |
| documentation notes that it is probably useful only when combined with (?{}) or |
| (??{}). Those are, of course, Perl features that are not present in PCRE2. The |
| nearest equivalent is the callout feature, as for example in this pattern: |
| .sp |
| a+(?C)(*FAIL) |
| .sp |
| A match with the string "aaaa" always fails, but the callout is taken before |
| each backtrack happens (in this example, 10 times). |
| .P |
| (*ACCEPT:NAME) and (*FAIL:NAME) behave exactly the same as |
| (*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively. |
| . |
| . |
| .SS "Recording which path was taken" |
| .rs |
| .sp |
| There is one verb whose main purpose is to track how a match was arrived at, |
| though it also has a secondary use in conjunction with advancing the match |
| starting point (see (*SKIP) below). |
| .sp |
| (*MARK:NAME) or (*:NAME) |
| .sp |
| A name is always required with this verb. There may be as many instances of |
| (*MARK) as you like in a pattern, and their names do not have to be unique. |
| .P |
| When a match succeeds, the name of the last-encountered (*MARK:NAME) on the |
| matching path is passed back to the caller as described in the section entitled |
| .\" HTML <a href="pcre2api.html#matchotherdata"> |
| .\" </a> |
| "Other information about the match" |
| .\" |
| in the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| documentation. This applies to all instances of (*MARK), including those inside |
| assertions and atomic groups. (There are differences in those cases when |
| (*MARK) is used in conjunction with (*SKIP) as described below.) |
| .P |
| As well as (*MARK), the (*COMMIT), (*PRUNE) and (*THEN) verbs may have |
| associated NAME arguments. Whichever is last on the matching path is passed |
| back. See below for more details of these other verbs. |
| .P |
| Here is an example of \fBpcre2test\fP output, where the "mark" modifier |
| requests the retrieval and outputting of (*MARK) data: |
| .sp |
| re> /X(*MARK:A)Y|X(*MARK:B)Z/mark |
| data> XY |
| 0: XY |
| MK: A |
| XZ |
| 0: XZ |
| MK: B |
| .sp |
| The (*MARK) name is tagged with "MK:" in this output, and in this example it |
| indicates which of the two alternatives matched. This is a more efficient way |
| of obtaining this information than putting each alternative in its own |
| capturing parentheses. |
| .P |
| If a verb with a name is encountered in a positive assertion that is true, the |
| name is recorded and passed back if it is the last-encountered. This does not |
| happen for negative assertions or failing positive assertions. |
| .P |
| After a partial match or a failed match, the last encountered name in the |
| entire match process is returned. For example: |
| .sp |
| re> /X(*MARK:A)Y|X(*MARK:B)Z/mark |
| data> XP |
| No match, mark = B |
| .sp |
| Note that in this unanchored example the mark is retained from the match |
| attempt that started at the letter "X" in the subject. Subsequent match |
| attempts starting at "P" and then with an empty string do not get as far as the |
| (*MARK) item, but nevertheless do not reset it. |
| .P |
| If you are interested in (*MARK) values after failed matches, you should |
| probably set the PCRE2_NO_START_OPTIMIZE option |
| .\" HTML <a href="#nooptimize"> |
| .\" </a> |
| (see above) |
| .\" |
| to ensure that the match is always attempted. |
| . |
| . |
| .SS "Verbs that act after backtracking" |
| .rs |
| .sp |
| The following verbs do nothing when they are encountered. Matching continues |
| with what follows, but if there is a subsequent match failure, causing a |
| backtrack to the verb, a failure is forced. That is, backtracking cannot pass |
| to the left of the verb. However, when one of these verbs appears inside an |
| atomic group or in a lookaround assertion that is true, its effect is confined |
| to that group, because once the group has been matched, there is never any |
| backtracking into it. Backtracking from beyond an assertion or an atomic group |
| ignores the entire group, and seeks a preceeding backtracking point. |
| .P |
| These verbs differ in exactly what kind of failure occurs when backtracking |
| reaches them. The behaviour described below is what happens when the verb is |
| not in a subroutine or an assertion. Subsequent sections cover these special |
| cases. |
| .sp |
| (*COMMIT) or (*COMMIT:NAME) |
| .sp |
| This verb causes the whole match to fail outright if there is a later matching |
| failure that causes backtracking to reach it. Even if the pattern is |
| unanchored, no further attempts to find a match by advancing the starting point |
| take place. If (*COMMIT) is the only backtracking verb that is encountered, |
| once it has been passed \fBpcre2_match()\fP is committed to finding a match at |
| the current starting point, or not at all. For example: |
| .sp |
| a+(*COMMIT)b |
| .sp |
| This matches "xxaab" but not "aacaab". It can be thought of as a kind of |
| dynamic anchor, or "I've started, so I must finish." |
| .P |
| The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COMMIT). It is |
| like (*MARK:NAME) in that the name is remembered for passing back to the |
| caller. However, (*SKIP:NAME) searches only for names set with (*MARK), |
| ignoring those set by (*COMMIT), (*PRUNE) and (*THEN). |
| .P |
| If there is more than one backtracking verb in a pattern, a different one that |
| follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a |
| match does not always guarantee that a match must be at this starting point. |
| .P |
| Note that (*COMMIT) at the start of a pattern is not the same as an anchor, |
| unless PCRE2's start-of-match optimizations are turned off, as shown in this |
| output from \fBpcre2test\fP: |
| .sp |
| re> /(*COMMIT)abc/ |
| data> xyzabc |
| 0: abc |
| data> |
| re> /(*COMMIT)abc/no_start_optimize |
| data> xyzabc |
| No match |
| .sp |
| For the first pattern, PCRE2 knows that any match must start with "a", so the |
| optimization skips along the subject to "a" before applying the pattern to the |
| first set of data. The match attempt then succeeds. The second pattern disables |
| the optimization that skips along to the first character. The pattern is now |
| applied starting at "x", and so the (*COMMIT) causes the match to fail without |
| trying any other starting points. |
| .sp |
| (*PRUNE) or (*PRUNE:NAME) |
| .sp |
| This verb causes the match to fail at the current starting position in the |
| subject if there is a later matching failure that causes backtracking to reach |
| it. If the pattern is unanchored, the normal "bumpalong" advance to the next |
| starting character then happens. Backtracking can occur as usual to the left of |
| (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but |
| if there is no match to the right, backtracking cannot cross (*PRUNE). In |
| simple cases, the use of (*PRUNE) is just an alternative to an atomic group or |
| possessive quantifier, but there are some uses of (*PRUNE) that cannot be |
| expressed in any other way. In an anchored pattern (*PRUNE) has the same effect |
| as (*COMMIT). |
| .P |
| The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE). It is |
| like (*MARK:NAME) in that the name is remembered for passing back to the |
| caller. However, (*SKIP:NAME) searches only for names set with (*MARK), |
| ignoring those set by (*COMMIT), (*PRUNE) or (*THEN). |
| .sp |
| (*SKIP) |
| .sp |
| This verb, when given without a name, is like (*PRUNE), except that if the |
| pattern is unanchored, the "bumpalong" advance is not to the next character, |
| but to the position in the subject where (*SKIP) was encountered. (*SKIP) |
| signifies that whatever text was matched leading up to it cannot be part of a |
| successful match if there is a later mismatch. Consider: |
| .sp |
| a+(*SKIP)b |
| .sp |
| If the subject is "aaaac...", after the first match attempt fails (starting at |
| the first character in the string), the starting point skips on to start the |
| next attempt at "c". Note that a possessive quantifer does not have the same |
| effect as this example; although it would suppress backtracking during the |
| first match attempt, the second attempt would start at the second character |
| instead of skipping on to "c". |
| .sp |
| (*SKIP:NAME) |
| .sp |
| When (*SKIP) has an associated name, its behaviour is modified. When such a |
| (*SKIP) is triggered, the previous path through the pattern is searched for the |
| most recent (*MARK) that has the same name. If one is found, the "bumpalong" |
| advance is to the subject position that corresponds to that (*MARK) instead of |
| to where (*SKIP) was encountered. If no (*MARK) with a matching name is found, |
| the (*SKIP) is ignored. |
| .P |
| The search for a (*MARK) name uses the normal backtracking mechanism, which |
| means that it does not see (*MARK) settings that are inside atomic groups or |
| assertions, because they are never re-entered by backtracking. Compare the |
| following \fBpcre2test\fP examples: |
| .sp |
| re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/ |
| data: abc |
| 0: a |
| 1: a |
| data: |
| re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/ |
| data: abc |
| 0: b |
| 1: b |
| .sp |
| In the first example, the (*MARK) setting is in an atomic group, so it is not |
| seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored. This allows |
| the second branch of the pattern to be tried at the first character position. |
| In the second example, the (*MARK) setting is not in an atomic group. This |
| allows (*SKIP:X) to find the (*MARK) when it backtracks, and this causes a new |
| matching attempt to start at the second character. This time, the (*MARK) is |
| never seen because "a" does not match "b", so the matcher immediately jumps to |
| the second branch of the pattern. |
| .P |
| Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores |
| names that are set by (*COMMIT:NAME), (*PRUNE:NAME) or (*THEN:NAME). |
| .sp |
| (*THEN) or (*THEN:NAME) |
| .sp |
| This verb causes a skip to the next innermost alternative when backtracking |
| reaches it. That is, it cancels any further backtracking within the current |
| alternative. Its name comes from the observation that it can be used for a |
| pattern-based if-then-else block: |
| .sp |
| ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... |
| .sp |
| If the COND1 pattern matches, FOO is tried (and possibly further items after |
| the end of the group if FOO succeeds); on failure, the matcher skips to the |
| second alternative and tries COND2, without backtracking into COND1. If that |
| succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no |
| more alternatives, so there is a backtrack to whatever came before the entire |
| group. If (*THEN) is not inside an alternation, it acts like (*PRUNE). |
| .P |
| The behaviour of (*THEN:NAME) is not the same as (*MARK:NAME)(*THEN). It is |
| like (*MARK:NAME) in that the name is remembered for passing back to the |
| caller. However, (*SKIP:NAME) searches only for names set with (*MARK), |
| ignoring those set by (*COMMIT), (*PRUNE) and (*THEN). |
| .P |
| A subpattern that does not contain a | character is just a part of the |
| enclosing alternative; it is not a nested alternation with only one |
| alternative. The effect of (*THEN) extends beyond such a subpattern to the |
| enclosing alternative. Consider this pattern, where A, B, etc. are complex |
| pattern fragments that do not contain any | characters at this level: |
| .sp |
| A (B(*THEN)C) | D |
| .sp |
| If A and B are matched, but there is a failure in C, matching does not |
| backtrack into A; instead it moves to the next alternative, that is, D. |
| However, if the subpattern containing (*THEN) is given an alternative, it |
| behaves differently: |
| .sp |
| A (B(*THEN)C | (*FAIL)) | D |
| .sp |
| The effect of (*THEN) is now confined to the inner subpattern. After a failure |
| in C, matching moves to (*FAIL), which causes the whole subpattern to fail |
| because there are no more alternatives to try. In this case, matching does now |
| backtrack into A. |
| .P |
| Note that a conditional subpattern is not considered as having two |
| alternatives, because only one is ever used. In other words, the | character in |
| a conditional subpattern has a different meaning. Ignoring white space, |
| consider: |
| .sp |
| ^.*? (?(?=a) a | b(*THEN)c ) |
| .sp |
| If the subject is "ba", this pattern does not match. Because .*? is ungreedy, |
| it initially matches zero characters. The condition (?=a) then fails, the |
| character "b" is matched, but "c" is not. At this point, matching does not |
| backtrack to .*? as might perhaps be expected from the presence of the | |
| character. The conditional subpattern is part of the single alternative that |
| comprises the whole pattern, and so the match fails. (If there was a backtrack |
| into .*?, allowing it to match "b", the match would succeed.) |
| .P |
| The verbs just described provide four different "strengths" of control when |
| subsequent matching fails. (*THEN) is the weakest, carrying on the match at the |
| next alternative. (*PRUNE) comes next, failing the match at the current |
| starting position, but allowing an advance to the next character (for an |
| unanchored pattern). (*SKIP) is similar, except that the advance may be more |
| than one character. (*COMMIT) is the strongest, causing the entire match to |
| fail. |
| . |
| . |
| .SS "More than one backtracking verb" |
| .rs |
| .sp |
| If more than one backtracking verb is present in a pattern, the one that is |
| backtracked onto first acts. For example, consider this pattern, where A, B, |
| etc. are complex pattern fragments: |
| .sp |
| (A(*COMMIT)B(*THEN)C|ABD) |
| .sp |
| If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to |
| fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes |
| the next alternative (ABD) to be tried. This behaviour is consistent, but is |
| not always the same as Perl's. It means that if two or more backtracking verbs |
| appear in succession, all the the last of them has no effect. Consider this |
| example: |
| .sp |
| ...(*COMMIT)(*PRUNE)... |
| .sp |
| If there is a matching failure to the right, backtracking onto (*PRUNE) causes |
| it to be triggered, and its action is taken. There can never be a backtrack |
| onto (*COMMIT). |
| . |
| . |
| .\" HTML <a name="btrepeat"></a> |
| .SS "Backtracking verbs in repeated groups" |
| .rs |
| .sp |
| PCRE2 sometimes differs from Perl in its handling of backtracking verbs in |
| repeated groups. For example, consider: |
| .sp |
| /(a(*COMMIT)b)+ac/ |
| .sp |
| If the subject is "abac", Perl matches unless its optimizations are disabled, |
| but PCRE2 always fails because the (*COMMIT) in the second repeat of the group |
| acts. |
| . |
| . |
| .\" HTML <a name="btassert"></a> |
| .SS "Backtracking verbs in assertions" |
| .rs |
| .sp |
| (*FAIL) in any assertion has its normal effect: it forces an immediate |
| backtrack. The behaviour of the other backtracking verbs depends on whether or |
| not the assertion is standalone or acting as the condition in a conditional |
| subpattern. |
| .P |
| (*ACCEPT) in a standalone positive assertion causes the assertion to succeed |
| without any further processing; captured strings and a (*MARK) name (if set) |
| are retained. In a standalone negative assertion, (*ACCEPT) causes the |
| assertion to fail without any further processing; captured substrings and any |
| (*MARK) name are discarded. |
| .P |
| If the assertion is a condition, (*ACCEPT) causes the condition to be true for |
| a positive assertion and false for a negative one; captured substrings are |
| retained in both cases. |
| .P |
| The remaining verbs act only when a later failure causes a backtrack to |
| reach them. This means that their effect is confined to the assertion, |
| because lookaround assertions are atomic. A backtrack that occurs after an |
| assertion is complete does not jump back into the assertion. Note in particular |
| that a (*MARK) name that is set in an assertion is not "seen" by an instance of |
| (*SKIP:NAME) latter in the pattern. |
| .P |
| The effect of (*THEN) is not allowed to escape beyond an assertion. If there |
| are no more branches to try, (*THEN) causes a positive assertion to be false, |
| and a negative assertion to be true. |
| .P |
| The other backtracking verbs are not treated specially if they appear in a |
| standalone positive assertion. In a conditional positive assertion, |
| backtracking (from within the assertion) into (*COMMIT), (*SKIP), or (*PRUNE) |
| causes the condition to be false. However, for both standalone and conditional |
| negative assertions, backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes |
| the assertion to be true, without considering any further alternative branches. |
| . |
| . |
| .\" HTML <a name="btsub"></a> |
| .SS "Backtracking verbs in subroutines" |
| .rs |
| .sp |
| These behaviours occur whether or not the subpattern is called recursively. |
| .P |
| (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to |
| succeed without any further processing. Matching then continues after the |
| subroutine call. Perl documents this behaviour. Perl's treatment of the other |
| verbs in subroutines is different in some cases. |
| .P |
| (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces |
| an immediate backtrack. |
| .P |
| (*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail when |
| triggered by being backtracked to in a subpattern called as a subroutine. There |
| is then a backtrack at the outer level. |
| .P |
| (*THEN), when triggered, skips to the next alternative in the innermost |
| enclosing group within the subpattern that has alternatives (its normal |
| behaviour). However, if there is no such group within the subroutine |
| subpattern, the subroutine match fails and there is a backtrack at the outer |
| level. |
| . |
| . |
| .SH "SEE ALSO" |
| .rs |
| .sp |
| \fBpcre2api\fP(3), \fBpcre2callout\fP(3), \fBpcre2matching\fP(3), |
| \fBpcre2syntax\fP(3), \fBpcre2\fP(3). |
| . |
| . |
| .SH AUTHOR |
| .rs |
| .sp |
| .nf |
| Philip Hazel |
| University Computing Service |
| Cambridge, England. |
| .fi |
| . |
| . |
| .SH REVISION |
| .rs |
| .sp |
| .nf |
| Last updated: 04 September 2018 |
| Copyright (c) 1997-2018 University of Cambridge. |
| .fi |