| :mod:`re` --- Regular expression operations |
| =========================================== |
| |
| .. module:: re |
| :synopsis: Regular expression operations. |
| .. moduleauthor:: Fredrik Lundh <fredrik@pythonware.com> |
| .. sectionauthor:: Andrew M. Kuchling <amk@amk.ca> |
| |
| |
| This module provides regular expression matching operations similar to |
| those found in Perl. |
| |
| Both patterns and strings to be searched can be Unicode strings as well as |
| 8-bit strings. However, Unicode strings and 8-bit strings cannot be mixed: |
| that is, you cannot match an Unicode string with a byte pattern or |
| vice-versa; similarly, when asking for a substitution, the replacement |
| string must be of the same type as both the pattern and the search string. |
| |
| Regular expressions use the backslash character (``'\'``) to indicate |
| special forms or to allow special characters to be used without invoking |
| their special meaning. This collides with Python's usage of the same |
| character for the same purpose in string literals; for example, to match |
| a literal backslash, one might have to write ``'\\\\'`` as the pattern |
| string, because the regular expression must be ``\\``, and each |
| backslash must be expressed as ``\\`` inside a regular Python string |
| literal. |
| |
| The solution is to use Python's raw string notation for regular expression |
| patterns; backslashes are not handled in any special way in a string literal |
| prefixed with ``'r'``. So ``r"\n"`` is a two-character string containing |
| ``'\'`` and ``'n'``, while ``"\n"`` is a one-character string containing a |
| newline. Usually patterns will be expressed in Python code using this raw |
| string notation. |
| |
| It is important to note that most regular expression operations are available as |
| module-level functions and methods on |
| :ref:`compiled regular expressions <re-objects>`. The functions are shortcuts |
| that don't require you to compile a regex object first, but miss some |
| fine-tuning parameters. |
| |
| .. seealso:: |
| |
| Mastering Regular Expressions |
| Book on regular expressions by Jeffrey Friedl, published by O'Reilly. The |
| second edition of the book no longer covers Python at all, but the first |
| edition covered writing good regular expression patterns in great detail. |
| |
| |
| .. _re-syntax: |
| |
| Regular Expression Syntax |
| ------------------------- |
| |
| A regular expression (or RE) specifies a set of strings that matches it; the |
| functions in this module let you check if a particular string matches a given |
| regular expression (or if a given regular expression matches a particular |
| string, which comes down to the same thing). |
| |
| Regular expressions can be concatenated to form new regular expressions; if *A* |
| and *B* are both regular expressions, then *AB* is also a regular expression. |
| In general, if a string *p* matches *A* and another string *q* matches *B*, the |
| string *pq* will match AB. This holds unless *A* or *B* contain low precedence |
| operations; boundary conditions between *A* and *B*; or have numbered group |
| references. Thus, complex expressions can easily be constructed from simpler |
| primitive expressions like the ones described here. For details of the theory |
| and implementation of regular expressions, consult the Friedl book referenced |
| above, or almost any textbook about compiler construction. |
| |
| A brief explanation of the format of regular expressions follows. For further |
| information and a gentler presentation, consult the :ref:`regex-howto`. |
| |
| Regular expressions can contain both special and ordinary characters. Most |
| ordinary characters, like ``'A'``, ``'a'``, or ``'0'``, are the simplest regular |
| expressions; they simply match themselves. You can concatenate ordinary |
| characters, so ``last`` matches the string ``'last'``. (In the rest of this |
| section, we'll write RE's in ``this special style``, usually without quotes, and |
| strings to be matched ``'in single quotes'``.) |
| |
| Some characters, like ``'|'`` or ``'('``, are special. Special |
| characters either stand for classes of ordinary characters, or affect |
| how the regular expressions around them are interpreted. Regular |
| expression pattern strings may not contain null bytes, but can specify |
| the null byte using a ``\number`` notation such as ``'\x00'``. |
| |
| |
| The special characters are: |
| |
| ``'.'`` |
| (Dot.) In the default mode, this matches any character except a newline. If |
| the :const:`DOTALL` flag has been specified, this matches any character |
| including a newline. |
| |
| ``'^'`` |
| (Caret.) Matches the start of the string, and in :const:`MULTILINE` mode also |
| matches immediately after each newline. |
| |
| ``'$'`` |
| Matches the end of the string or just before the newline at the end of the |
| string, and in :const:`MULTILINE` mode also matches before a newline. ``foo`` |
| matches both 'foo' and 'foobar', while the regular expression ``foo$`` matches |
| only 'foo'. More interestingly, searching for ``foo.$`` in ``'foo1\nfoo2\n'`` |
| matches 'foo2' normally, but 'foo1' in :const:`MULTILINE` mode; searching for |
| a single ``$`` in ``'foo\n'`` will find two (empty) matches: one just before |
| the newline, and one at the end of the string. |
| |
| ``'*'`` |
| Causes the resulting RE to match 0 or more repetitions of the preceding RE, as |
| many repetitions as are possible. ``ab*`` will match 'a', 'ab', or 'a' followed |
| by any number of 'b's. |
| |
| ``'+'`` |
| Causes the resulting RE to match 1 or more repetitions of the preceding RE. |
| ``ab+`` will match 'a' followed by any non-zero number of 'b's; it will not |
| match just 'a'. |
| |
| ``'?'`` |
| Causes the resulting RE to match 0 or 1 repetitions of the preceding RE. |
| ``ab?`` will match either 'a' or 'ab'. |
| |
| ``*?``, ``+?``, ``??`` |
| The ``'*'``, ``'+'``, and ``'?'`` qualifiers are all :dfn:`greedy`; they match |
| as much text as possible. Sometimes this behaviour isn't desired; if the RE |
| ``<.*>`` is matched against ``'<H1>title</H1>'``, it will match the entire |
| string, and not just ``'<H1>'``. Adding ``'?'`` after the qualifier makes it |
| perform the match in :dfn:`non-greedy` or :dfn:`minimal` fashion; as *few* |
| characters as possible will be matched. Using ``.*?`` in the previous |
| expression will match only ``'<H1>'``. |
| |
| ``{m}`` |
| Specifies that exactly *m* copies of the previous RE should be matched; fewer |
| matches cause the entire RE not to match. For example, ``a{6}`` will match |
| exactly six ``'a'`` characters, but not five. |
| |
| ``{m,n}`` |
| Causes the resulting RE to match from *m* to *n* repetitions of the preceding |
| RE, attempting to match as many repetitions as possible. For example, |
| ``a{3,5}`` will match from 3 to 5 ``'a'`` characters. Omitting *m* specifies a |
| lower bound of zero, and omitting *n* specifies an infinite upper bound. As an |
| example, ``a{4,}b`` will match ``aaaab`` or a thousand ``'a'`` characters |
| followed by a ``b``, but not ``aaab``. The comma may not be omitted or the |
| modifier would be confused with the previously described form. |
| |
| ``{m,n}?`` |
| Causes the resulting RE to match from *m* to *n* repetitions of the preceding |
| RE, attempting to match as *few* repetitions as possible. This is the |
| non-greedy version of the previous qualifier. For example, on the |
| 6-character string ``'aaaaaa'``, ``a{3,5}`` will match 5 ``'a'`` characters, |
| while ``a{3,5}?`` will only match 3 characters. |
| |
| ``'\'`` |
| Either escapes special characters (permitting you to match characters like |
| ``'*'``, ``'?'``, and so forth), or signals a special sequence; special |
| sequences are discussed below. |
| |
| If you're not using a raw string to express the pattern, remember that Python |
| also uses the backslash as an escape sequence in string literals; if the escape |
| sequence isn't recognized by Python's parser, the backslash and subsequent |
| character are included in the resulting string. However, if Python would |
| recognize the resulting sequence, the backslash should be repeated twice. This |
| is complicated and hard to understand, so it's highly recommended that you use |
| raw strings for all but the simplest expressions. |
| |
| ``[]`` |
| Used to indicate a set of characters. In a set: |
| |
| * Characters can be listed individually, e.g. ``[amk]`` will match ``'a'``, |
| ``'m'``, or ``'k'``. |
| |
| * Ranges of characters can be indicated by giving two characters and separating |
| them by a ``'-'``, for example ``[a-z]`` will match any lowercase ASCII letter, |
| ``[0-5][0-9]`` will match all the two-digits numbers from ``00`` to ``59``, and |
| ``[0-9A-Fa-f]`` will match any hexadecimal digit. If ``-`` is escaped (e.g. |
| ``[a\-z]``) or if it's placed as the first or last character (e.g. ``[a-]``), |
| it will match a literal ``'-'``. |
| |
| * Special characters lose their special meaning inside sets. For example, |
| ``[(+*)]`` will match any of the literal characters ``'('``, ``'+'``, |
| ``'*'``, or ``')'``. |
| |
| * Character classes such as ``\w`` or ``\S`` (defined below) are also accepted |
| inside a set, although the characters they match depends on whether |
| :const:`ASCII` or :const:`LOCALE` mode is in force. |
| |
| * Characters that are not within a range can be matched by :dfn:`complementing` |
| the set. If the first character of the set is ``'^'``, all the characters |
| that are *not* in the set will be matched. For example, ``[^5]`` will match |
| any character except ``'5'``, and ``[^^]`` will match any character except |
| ``'^'``. ``^`` has no special meaning if it's not the first character in |
| the set. |
| |
| * To match a literal ``']'`` inside a set, precede it with a backslash, or |
| place it at the beginning of the set. For example, both ``[()[\]{}]`` and |
| ``[]()[{}]`` will both match a parenthesis. |
| |
| ``'|'`` |
| ``A|B``, where A and B can be arbitrary REs, creates a regular expression that |
| will match either A or B. An arbitrary number of REs can be separated by the |
| ``'|'`` in this way. This can be used inside groups (see below) as well. As |
| the target string is scanned, REs separated by ``'|'`` are tried from left to |
| right. When one pattern completely matches, that branch is accepted. This means |
| that once ``A`` matches, ``B`` will not be tested further, even if it would |
| produce a longer overall match. In other words, the ``'|'`` operator is never |
| greedy. To match a literal ``'|'``, use ``\|``, or enclose it inside a |
| character class, as in ``[|]``. |
| |
| ``(...)`` |
| Matches whatever regular expression is inside the parentheses, and indicates the |
| start and end of a group; the contents of a group can be retrieved after a match |
| has been performed, and can be matched later in the string with the ``\number`` |
| special sequence, described below. To match the literals ``'('`` or ``')'``, |
| use ``\(`` or ``\)``, or enclose them inside a character class: ``[(] [)]``. |
| |
| ``(?...)`` |
| This is an extension notation (a ``'?'`` following a ``'('`` is not meaningful |
| otherwise). The first character after the ``'?'`` determines what the meaning |
| and further syntax of the construct is. Extensions usually do not create a new |
| group; ``(?P<name>...)`` is the only exception to this rule. Following are the |
| currently supported extensions. |
| |
| ``(?aiLmsux)`` |
| (One or more letters from the set ``'a'``, ``'i'``, ``'L'``, ``'m'``, |
| ``'s'``, ``'u'``, ``'x'``.) The group matches the empty string; the |
| letters set the corresponding flags: :const:`re.A` (ASCII-only matching), |
| :const:`re.I` (ignore case), :const:`re.L` (locale dependent), |
| :const:`re.M` (multi-line), :const:`re.S` (dot matches all), |
| and :const:`re.X` (verbose), for the entire regular expression. (The |
| flags are described in :ref:`contents-of-module-re`.) This |
| is useful if you wish to include the flags as part of the regular |
| expression, instead of passing a *flag* argument to the |
| :func:`re.compile` function. |
| |
| Note that the ``(?x)`` flag changes how the expression is parsed. It should be |
| used first in the expression string, or after one or more whitespace characters. |
| If there are non-whitespace characters before the flag, the results are |
| undefined. |
| |
| ``(?:...)`` |
| A non-capturing version of regular parentheses. Matches whatever regular |
| expression is inside the parentheses, but the substring matched by the group |
| *cannot* be retrieved after performing a match or referenced later in the |
| pattern. |
| |
| ``(?P<name>...)`` |
| Similar to regular parentheses, but the substring matched by the group is |
| accessible via the symbolic group name *name*. Group names must be valid |
| Python identifiers, and each group name must be defined only once within a |
| regular expression. A symbolic group is also a numbered group, just as if |
| the group were not named. |
| |
| Named groups can be referenced in three contexts. If the pattern is |
| ``(?P<quote>['"]).*?(?P=quote)`` (i.e. matching a string quoted with either |
| single or double quotes): |
| |
| +---------------------------------------+----------------------------------+ |
| | Context of reference to group "quote" | Ways to reference it | |
| +=======================================+==================================+ |
| | in the same pattern itself | * ``(?P=quote)`` (as shown) | |
| | | * ``\1`` | |
| +---------------------------------------+----------------------------------+ |
| | when processing match object ``m`` | * ``m.group('quote')`` | |
| | | * ``m.end('quote')`` (etc.) | |
| +---------------------------------------+----------------------------------+ |
| | in a string passed to the ``repl`` | * ``\g<quote>`` | |
| | argument of ``re.sub()`` | * ``\g<1>`` | |
| | | * ``\1`` | |
| +---------------------------------------+----------------------------------+ |
| |
| ``(?P=name)`` |
| A backreference to a named group; it matches whatever text was matched by the |
| earlier group named *name*. |
| |
| ``(?#...)`` |
| A comment; the contents of the parentheses are simply ignored. |
| |
| ``(?=...)`` |
| Matches if ``...`` matches next, but doesn't consume any of the string. This is |
| called a lookahead assertion. For example, ``Isaac (?=Asimov)`` will match |
| ``'Isaac '`` only if it's followed by ``'Asimov'``. |
| |
| ``(?!...)`` |
| Matches if ``...`` doesn't match next. This is a negative lookahead assertion. |
| For example, ``Isaac (?!Asimov)`` will match ``'Isaac '`` only if it's *not* |
| followed by ``'Asimov'``. |
| |
| ``(?<=...)`` |
| Matches if the current position in the string is preceded by a match for ``...`` |
| that ends at the current position. This is called a :dfn:`positive lookbehind |
| assertion`. ``(?<=abc)def`` will find a match in ``abcdef``, since the |
| lookbehind will back up 3 characters and check if the contained pattern matches. |
| The contained pattern must only match strings of some fixed length, meaning that |
| ``abc`` or ``a|b`` are allowed, but ``a*`` and ``a{3,4}`` are not. Note that |
| patterns which start with positive lookbehind assertions will not match at the |
| beginning of the string being searched; you will most likely want to use the |
| :func:`search` function rather than the :func:`match` function: |
| |
| >>> import re |
| >>> m = re.search('(?<=abc)def', 'abcdef') |
| >>> m.group(0) |
| 'def' |
| |
| This example looks for a word following a hyphen: |
| |
| >>> m = re.search('(?<=-)\w+', 'spam-egg') |
| >>> m.group(0) |
| 'egg' |
| |
| ``(?<!...)`` |
| Matches if the current position in the string is not preceded by a match for |
| ``...``. This is called a :dfn:`negative lookbehind assertion`. Similar to |
| positive lookbehind assertions, the contained pattern must only match strings of |
| some fixed length. Patterns which start with negative lookbehind assertions may |
| match at the beginning of the string being searched. |
| |
| ``(?(id/name)yes-pattern|no-pattern)`` |
| Will try to match with ``yes-pattern`` if the group with given *id* or |
| *name* exists, and with ``no-pattern`` if it doesn't. ``no-pattern`` is |
| optional and can be omitted. For example, |
| ``(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)`` is a poor email matching pattern, which |
| will match with ``'<user@host.com>'`` as well as ``'user@host.com'``, but |
| not with ``'<user@host.com'`` nor ``'user@host.com>'``. |
| |
| |
| The special sequences consist of ``'\'`` and a character from the list below. |
| If the ordinary character is not on the list, then the resulting RE will match |
| the second character. For example, ``\$`` matches the character ``'$'``. |
| |
| ``\number`` |
| Matches the contents of the group of the same number. Groups are numbered |
| starting from 1. For example, ``(.+) \1`` matches ``'the the'`` or ``'55 55'``, |
| but not ``'thethe'`` (note the space after the group). This special sequence |
| can only be used to match one of the first 99 groups. If the first digit of |
| *number* is 0, or *number* is 3 octal digits long, it will not be interpreted as |
| a group match, but as the character with octal value *number*. Inside the |
| ``'['`` and ``']'`` of a character class, all numeric escapes are treated as |
| characters. |
| |
| ``\A`` |
| Matches only at the start of the string. |
| |
| ``\b`` |
| Matches the empty string, but only at the beginning or end of a word. |
| A word is defined as a sequence of Unicode alphanumeric or underscore |
| characters, so the end of a word is indicated by whitespace or a |
| non-alphanumeric, non-underscore Unicode character. Note that formally, |
| ``\b`` is defined as the boundary between a ``\w`` and a ``\W`` character |
| (or vice versa), or between ``\w`` and the beginning/end of the string. |
| This means that ``r'\bfoo\b'`` matches ``'foo'``, ``'foo.'``, ``'(foo)'``, |
| ``'bar foo baz'`` but not ``'foobar'`` or ``'foo3'``. |
| |
| By default Unicode alphanumerics are the ones used, but this can be changed |
| by using the :const:`ASCII` flag. Inside a character range, ``\b`` |
| represents the backspace character, for compatibility with Python's string |
| literals. |
| |
| ``\B`` |
| Matches the empty string, but only when it is *not* at the beginning or end |
| of a word. This means that ``r'py\B'`` matches ``'python'``, ``'py3'``, |
| ``'py2'``, but not ``'py'``, ``'py.'``, or ``'py!'``. |
| ``\B`` is just the opposite of ``\b``, so word characters are |
| Unicode alphanumerics or the underscore, although this can be changed |
| by using the :const:`ASCII` flag. |
| |
| ``\d`` |
| For Unicode (str) patterns: |
| Matches any Unicode decimal digit (that is, any character in |
| Unicode character category [Nd]). This includes ``[0-9]``, and |
| also many other digit characters. If the :const:`ASCII` flag is |
| used only ``[0-9]`` is matched (but the flag affects the entire |
| regular expression, so in such cases using an explicit ``[0-9]`` |
| may be a better choice). |
| For 8-bit (bytes) patterns: |
| Matches any decimal digit; this is equivalent to ``[0-9]``. |
| |
| ``\D`` |
| Matches any character which is not a Unicode decimal digit. This is |
| the opposite of ``\d``. If the :const:`ASCII` flag is used this |
| becomes the equivalent of ``[^0-9]`` (but the flag affects the entire |
| regular expression, so in such cases using an explicit ``[^0-9]`` may |
| be a better choice). |
| |
| ``\s`` |
| For Unicode (str) patterns: |
| Matches Unicode whitespace characters (which includes |
| ``[ \t\n\r\f\v]``, and also many other characters, for example the |
| non-breaking spaces mandated by typography rules in many |
| languages). If the :const:`ASCII` flag is used, only |
| ``[ \t\n\r\f\v]`` is matched (but the flag affects the entire |
| regular expression, so in such cases using an explicit |
| ``[ \t\n\r\f\v]`` may be a better choice). |
| |
| For 8-bit (bytes) patterns: |
| Matches characters considered whitespace in the ASCII character set; |
| this is equivalent to ``[ \t\n\r\f\v]``. |
| |
| ``\S`` |
| Matches any character which is not a Unicode whitespace character. This is |
| the opposite of ``\s``. If the :const:`ASCII` flag is used this |
| becomes the equivalent of ``[^ \t\n\r\f\v]`` (but the flag affects the entire |
| regular expression, so in such cases using an explicit ``[^ \t\n\r\f\v]`` may |
| be a better choice). |
| |
| ``\w`` |
| For Unicode (str) patterns: |
| Matches Unicode word characters; this includes most characters |
| that can be part of a word in any language, as well as numbers and |
| the underscore. If the :const:`ASCII` flag is used, only |
| ``[a-zA-Z0-9_]`` is matched (but the flag affects the entire |
| regular expression, so in such cases using an explicit |
| ``[a-zA-Z0-9_]`` may be a better choice). |
| For 8-bit (bytes) patterns: |
| Matches characters considered alphanumeric in the ASCII character set; |
| this is equivalent to ``[a-zA-Z0-9_]``. |
| |
| ``\W`` |
| Matches any character which is not a Unicode word character. This is |
| the opposite of ``\w``. If the :const:`ASCII` flag is used this |
| becomes the equivalent of ``[^a-zA-Z0-9_]`` (but the flag affects the |
| entire regular expression, so in such cases using an explicit |
| ``[^a-zA-Z0-9_]`` may be a better choice). |
| |
| ``\Z`` |
| Matches only at the end of the string. |
| |
| Most of the standard escapes supported by Python string literals are also |
| accepted by the regular expression parser:: |
| |
| \a \b \f \n |
| \r \t \u \U |
| \v \x \\ |
| |
| (Note that ``\b`` is used to represent word boundaries, and means "backspace" |
| only inside character classes.) |
| |
| ``'\u'`` and ``'\U'`` escape sequences are only recognized in Unicode |
| patterns. In bytes patterns they are not treated specially. |
| |
| Octal escapes are included in a limited form. If the first digit is a 0, or if |
| there are three octal digits, it is considered an octal escape. Otherwise, it is |
| a group reference. As for string literals, octal escapes are always at most |
| three digits in length. |
| |
| .. versionchanged:: 3.3 |
| The ``'\u'`` and ``'\U'`` escape sequences have been added. |
| |
| |
| |
| .. _contents-of-module-re: |
| |
| Module Contents |
| --------------- |
| |
| The module defines several functions, constants, and an exception. Some of the |
| functions are simplified versions of the full featured methods for compiled |
| regular expressions. Most non-trivial applications always use the compiled |
| form. |
| |
| |
| .. function:: compile(pattern, flags=0) |
| |
| Compile a regular expression pattern into a regular expression object, which |
| can be used for matching using its :func:`~regex.match` and |
| :func:`~regex.search` methods, described below. |
| |
| The expression's behaviour can be modified by specifying a *flags* value. |
| Values can be any of the following variables, combined using bitwise OR (the |
| ``|`` operator). |
| |
| The sequence :: |
| |
| prog = re.compile(pattern) |
| result = prog.match(string) |
| |
| is equivalent to :: |
| |
| result = re.match(pattern, string) |
| |
| but using :func:`re.compile` and saving the resulting regular expression |
| object for reuse is more efficient when the expression will be used several |
| times in a single program. |
| |
| .. note:: |
| |
| The compiled versions of the most recent patterns passed to |
| :func:`re.compile` and the module-level matching functions are cached, so |
| programs that use only a few regular expressions at a time needn't worry |
| about compiling regular expressions. |
| |
| |
| .. data:: A |
| ASCII |
| |
| Make ``\w``, ``\W``, ``\b``, ``\B``, ``\d``, ``\D``, ``\s`` and ``\S`` |
| perform ASCII-only matching instead of full Unicode matching. This is only |
| meaningful for Unicode patterns, and is ignored for byte patterns. |
| |
| Note that for backward compatibility, the :const:`re.U` flag still |
| exists (as well as its synonym :const:`re.UNICODE` and its embedded |
| counterpart ``(?u)``), but these are redundant in Python 3 since |
| matches are Unicode by default for strings (and Unicode matching |
| isn't allowed for bytes). |
| |
| |
| .. data:: DEBUG |
| |
| Display debug information about compiled expression. |
| |
| |
| .. data:: I |
| IGNORECASE |
| |
| Perform case-insensitive matching; expressions like ``[A-Z]`` will match |
| lowercase letters, too. This is not affected by the current locale |
| and works for Unicode characters as expected. |
| |
| |
| .. data:: L |
| LOCALE |
| |
| Make ``\w``, ``\W``, ``\b``, ``\B``, ``\s`` and ``\S`` dependent on the |
| current locale. The use of this flag is discouraged as the locale mechanism |
| is very unreliable, and it only handles one "culture" at a time anyway; |
| you should use Unicode matching instead, which is the default in Python 3 |
| for Unicode (str) patterns. |
| |
| |
| .. data:: M |
| MULTILINE |
| |
| When specified, the pattern character ``'^'`` matches at the beginning of the |
| string and at the beginning of each line (immediately following each newline); |
| and the pattern character ``'$'`` matches at the end of the string and at the |
| end of each line (immediately preceding each newline). By default, ``'^'`` |
| matches only at the beginning of the string, and ``'$'`` only at the end of the |
| string and immediately before the newline (if any) at the end of the string. |
| |
| |
| .. data:: S |
| DOTALL |
| |
| Make the ``'.'`` special character match any character at all, including a |
| newline; without this flag, ``'.'`` will match anything *except* a newline. |
| |
| |
| .. data:: X |
| VERBOSE |
| |
| This flag allows you to write regular expressions that look nicer. Whitespace |
| within the pattern is ignored, except when in a character class or preceded by |
| an unescaped backslash, and, when a line contains a ``'#'`` neither in a |
| character class or preceded by an unescaped backslash, all characters from the |
| leftmost such ``'#'`` through the end of the line are ignored. |
| |
| That means that the two following regular expression objects that match a |
| decimal number are functionally equal:: |
| |
| a = re.compile(r"""\d + # the integral part |
| \. # the decimal point |
| \d * # some fractional digits""", re.X) |
| b = re.compile(r"\d+\.\d*") |
| |
| |
| |
| |
| .. function:: search(pattern, string, flags=0) |
| |
| Scan through *string* looking for the first location where the regular expression |
| *pattern* produces a match, and return a corresponding :ref:`match object |
| <match-objects>`. Return ``None`` if no position in the string matches the |
| pattern; note that this is different from finding a zero-length match at some |
| point in the string. |
| |
| |
| .. function:: match(pattern, string, flags=0) |
| |
| If zero or more characters at the beginning of *string* match the regular |
| expression *pattern*, return a corresponding :ref:`match object |
| <match-objects>`. Return ``None`` if the string does not match the pattern; |
| note that this is different from a zero-length match. |
| |
| Note that even in :const:`MULTILINE` mode, :func:`re.match` will only match |
| at the beginning of the string and not at the beginning of each line. |
| |
| If you want to locate a match anywhere in *string*, use :func:`search` |
| instead (see also :ref:`search-vs-match`). |
| |
| |
| .. function:: fullmatch(pattern, string, flags=0) |
| |
| If the whole *string* matches the regular expression *pattern*, return a |
| corresponding :ref:`match object <match-objects>`. Return ``None`` if the |
| string does not match the pattern; note that this is different from a |
| zero-length match. |
| |
| .. versionadded:: 3.4 |
| |
| |
| .. function:: split(pattern, string, maxsplit=0, flags=0) |
| |
| Split *string* by the occurrences of *pattern*. If capturing parentheses are |
| used in *pattern*, then the text of all groups in the pattern are also returned |
| as part of the resulting list. If *maxsplit* is nonzero, at most *maxsplit* |
| splits occur, and the remainder of the string is returned as the final element |
| of the list. :: |
| |
| >>> re.split('\W+', 'Words, words, words.') |
| ['Words', 'words', 'words', ''] |
| >>> re.split('(\W+)', 'Words, words, words.') |
| ['Words', ', ', 'words', ', ', 'words', '.', ''] |
| >>> re.split('\W+', 'Words, words, words.', 1) |
| ['Words', 'words, words.'] |
| >>> re.split('[a-f]+', '0a3B9', flags=re.IGNORECASE) |
| ['0', '3', '9'] |
| |
| If there are capturing groups in the separator and it matches at the start of |
| the string, the result will start with an empty string. The same holds for |
| the end of the string: |
| |
| >>> re.split('(\W+)', '...words, words...') |
| ['', '...', 'words', ', ', 'words', '...', ''] |
| |
| That way, separator components are always found at the same relative |
| indices within the result list. |
| |
| Note that *split* will never split a string on an empty pattern match. |
| For example: |
| |
| >>> re.split('x*', 'foo') |
| ['foo'] |
| >>> re.split("(?m)^$", "foo\n\nbar\n") |
| ['foo\n\nbar\n'] |
| |
| .. versionchanged:: 3.1 |
| Added the optional flags argument. |
| |
| |
| .. function:: findall(pattern, string, flags=0) |
| |
| Return all non-overlapping matches of *pattern* in *string*, as a list of |
| strings. The *string* is scanned left-to-right, and matches are returned in |
| the order found. If one or more groups are present in the pattern, return a |
| list of groups; this will be a list of tuples if the pattern has more than |
| one group. Empty matches are included in the result unless they touch the |
| beginning of another match. |
| |
| |
| .. function:: finditer(pattern, string, flags=0) |
| |
| Return an :term:`iterator` yielding :ref:`match objects <match-objects>` over |
| all non-overlapping matches for the RE *pattern* in *string*. The *string* |
| is scanned left-to-right, and matches are returned in the order found. Empty |
| matches are included in the result unless they touch the beginning of another |
| match. |
| |
| |
| .. function:: sub(pattern, repl, string, count=0, flags=0) |
| |
| Return the string obtained by replacing the leftmost non-overlapping occurrences |
| of *pattern* in *string* by the replacement *repl*. If the pattern isn't found, |
| *string* is returned unchanged. *repl* can be a string or a function; if it is |
| a string, any backslash escapes in it are processed. That is, ``\n`` is |
| converted to a single newline character, ``\r`` is converted to a carriage return, and |
| so forth. Unknown escapes such as ``\j`` are left alone. Backreferences, such |
| as ``\6``, are replaced with the substring matched by group 6 in the pattern. |
| For example: |
| |
| >>> re.sub(r'def\s+([a-zA-Z_][a-zA-Z_0-9]*)\s*\(\s*\):', |
| ... r'static PyObject*\npy_\1(void)\n{', |
| ... 'def myfunc():') |
| 'static PyObject*\npy_myfunc(void)\n{' |
| |
| If *repl* is a function, it is called for every non-overlapping occurrence of |
| *pattern*. The function takes a single match object argument, and returns the |
| replacement string. For example: |
| |
| >>> def dashrepl(matchobj): |
| ... if matchobj.group(0) == '-': return ' ' |
| ... else: return '-' |
| >>> re.sub('-{1,2}', dashrepl, 'pro----gram-files') |
| 'pro--gram files' |
| >>> re.sub(r'\sAND\s', ' & ', 'Baked Beans And Spam', flags=re.IGNORECASE) |
| 'Baked Beans & Spam' |
| |
| The pattern may be a string or an RE object. |
| |
| The optional argument *count* is the maximum number of pattern occurrences to be |
| replaced; *count* must be a non-negative integer. If omitted or zero, all |
| occurrences will be replaced. Empty matches for the pattern are replaced only |
| when not adjacent to a previous match, so ``sub('x*', '-', 'abc')`` returns |
| ``'-a-b-c-'``. |
| |
| In string-type *repl* arguments, in addition to the character escapes and |
| backreferences described above, |
| ``\g<name>`` will use the substring matched by the group named ``name``, as |
| defined by the ``(?P<name>...)`` syntax. ``\g<number>`` uses the corresponding |
| group number; ``\g<2>`` is therefore equivalent to ``\2``, but isn't ambiguous |
| in a replacement such as ``\g<2>0``. ``\20`` would be interpreted as a |
| reference to group 20, not a reference to group 2 followed by the literal |
| character ``'0'``. The backreference ``\g<0>`` substitutes in the entire |
| substring matched by the RE. |
| |
| .. versionchanged:: 3.1 |
| Added the optional flags argument. |
| |
| |
| .. function:: subn(pattern, repl, string, count=0, flags=0) |
| |
| Perform the same operation as :func:`sub`, but return a tuple ``(new_string, |
| number_of_subs_made)``. |
| |
| .. versionchanged:: 3.1 |
| Added the optional flags argument. |
| |
| |
| .. function:: escape(string) |
| |
| Escape all the characters in pattern except ASCII letters, numbers and ``'_'``. |
| This is useful if you want to match an arbitrary literal string that may |
| have regular expression metacharacters in it. |
| |
| .. versionchanged:: 3.3 |
| The ``'_'`` character is no longer escaped. |
| |
| |
| .. function:: purge() |
| |
| Clear the regular expression cache. |
| |
| |
| .. exception:: error |
| |
| Exception raised when a string passed to one of the functions here is not a |
| valid regular expression (for example, it might contain unmatched parentheses) |
| or when some other error occurs during compilation or matching. It is never an |
| error if a string contains no match for a pattern. |
| |
| |
| .. _re-objects: |
| |
| Regular Expression Objects |
| -------------------------- |
| |
| Compiled regular expression objects support the following methods and |
| attributes: |
| |
| .. method:: regex.search(string[, pos[, endpos]]) |
| |
| Scan through *string* looking for a location where this regular expression |
| produces a match, and return a corresponding :ref:`match object |
| <match-objects>`. Return ``None`` if no position in the string matches the |
| pattern; note that this is different from finding a zero-length match at some |
| point in the string. |
| |
| The optional second parameter *pos* gives an index in the string where the |
| search is to start; it defaults to ``0``. This is not completely equivalent to |
| slicing the string; the ``'^'`` pattern character matches at the real beginning |
| of the string and at positions just after a newline, but not necessarily at the |
| index where the search is to start. |
| |
| The optional parameter *endpos* limits how far the string will be searched; it |
| will be as if the string is *endpos* characters long, so only the characters |
| from *pos* to ``endpos - 1`` will be searched for a match. If *endpos* is less |
| than *pos*, no match will be found; otherwise, if *rx* is a compiled regular |
| expression object, ``rx.search(string, 0, 50)`` is equivalent to |
| ``rx.search(string[:50], 0)``. |
| |
| >>> pattern = re.compile("d") |
| >>> pattern.search("dog") # Match at index 0 |
| <_sre.SRE_Match object; span=(0, 1), match='d'> |
| >>> pattern.search("dog", 1) # No match; search doesn't include the "d" |
| |
| |
| .. method:: regex.match(string[, pos[, endpos]]) |
| |
| If zero or more characters at the *beginning* of *string* match this regular |
| expression, return a corresponding :ref:`match object <match-objects>`. |
| Return ``None`` if the string does not match the pattern; note that this is |
| different from a zero-length match. |
| |
| The optional *pos* and *endpos* parameters have the same meaning as for the |
| :meth:`~regex.search` method. |
| |
| >>> pattern = re.compile("o") |
| >>> pattern.match("dog") # No match as "o" is not at the start of "dog". |
| >>> pattern.match("dog", 1) # Match as "o" is the 2nd character of "dog". |
| <_sre.SRE_Match object; span=(1, 2), match='o'> |
| |
| If you want to locate a match anywhere in *string*, use |
| :meth:`~regex.search` instead (see also :ref:`search-vs-match`). |
| |
| |
| .. method:: regex.fullmatch(string[, pos[, endpos]]) |
| |
| If the whole *string* matches this regular expression, return a corresponding |
| :ref:`match object <match-objects>`. Return ``None`` if the string does not |
| match the pattern; note that this is different from a zero-length match. |
| |
| The optional *pos* and *endpos* parameters have the same meaning as for the |
| :meth:`~regex.search` method. |
| |
| >>> pattern = re.compile("o[gh]") |
| >>> pattern.fullmatch("dog") # No match as "o" is not at the start of "dog". |
| >>> pattern.fullmatch("ogre") # No match as not the full string matches. |
| >>> pattern.fullmatch("doggie", 1, 3) # Matches within given limits. |
| <_sre.SRE_Match object; span=(1, 3), match='og'> |
| |
| .. versionadded:: 3.4 |
| |
| |
| .. method:: regex.split(string, maxsplit=0) |
| |
| Identical to the :func:`split` function, using the compiled pattern. |
| |
| |
| .. method:: regex.findall(string[, pos[, endpos]]) |
| |
| Similar to the :func:`findall` function, using the compiled pattern, but |
| also accepts optional *pos* and *endpos* parameters that limit the search |
| region like for :meth:`match`. |
| |
| |
| .. method:: regex.finditer(string[, pos[, endpos]]) |
| |
| Similar to the :func:`finditer` function, using the compiled pattern, but |
| also accepts optional *pos* and *endpos* parameters that limit the search |
| region like for :meth:`match`. |
| |
| |
| .. method:: regex.sub(repl, string, count=0) |
| |
| Identical to the :func:`sub` function, using the compiled pattern. |
| |
| |
| .. method:: regex.subn(repl, string, count=0) |
| |
| Identical to the :func:`subn` function, using the compiled pattern. |
| |
| |
| .. attribute:: regex.flags |
| |
| The regex matching flags. This is a combination of the flags given to |
| :func:`.compile`, any ``(?...)`` inline flags in the pattern, and implicit |
| flags such as :data:`UNICODE` if the pattern is a Unicode string. |
| |
| |
| .. attribute:: regex.groups |
| |
| The number of capturing groups in the pattern. |
| |
| |
| .. attribute:: regex.groupindex |
| |
| A dictionary mapping any symbolic group names defined by ``(?P<id>)`` to group |
| numbers. The dictionary is empty if no symbolic groups were used in the |
| pattern. |
| |
| |
| .. attribute:: regex.pattern |
| |
| The pattern string from which the RE object was compiled. |
| |
| |
| .. _match-objects: |
| |
| Match Objects |
| ------------- |
| |
| Match objects always have a boolean value of ``True``. |
| Since :meth:`~regex.match` and :meth:`~regex.search` return ``None`` |
| when there is no match, you can test whether there was a match with a simple |
| ``if`` statement:: |
| |
| match = re.search(pattern, string) |
| if match: |
| process(match) |
| |
| Match objects support the following methods and attributes: |
| |
| |
| .. method:: match.expand(template) |
| |
| Return the string obtained by doing backslash substitution on the template |
| string *template*, as done by the :meth:`~regex.sub` method. |
| Escapes such as ``\n`` are converted to the appropriate characters, |
| and numeric backreferences (``\1``, ``\2``) and named backreferences |
| (``\g<1>``, ``\g<name>``) are replaced by the contents of the |
| corresponding group. |
| |
| |
| .. method:: match.group([group1, ...]) |
| |
| Returns one or more subgroups of the match. If there is a single argument, the |
| result is a single string; if there are multiple arguments, the result is a |
| tuple with one item per argument. Without arguments, *group1* defaults to zero |
| (the whole match is returned). If a *groupN* argument is zero, the corresponding |
| return value is the entire matching string; if it is in the inclusive range |
| [1..99], it is the string matching the corresponding parenthesized group. If a |
| group number is negative or larger than the number of groups defined in the |
| pattern, an :exc:`IndexError` exception is raised. If a group is contained in a |
| part of the pattern that did not match, the corresponding result is ``None``. |
| If a group is contained in a part of the pattern that matched multiple times, |
| the last match is returned. |
| |
| >>> m = re.match(r"(\w+) (\w+)", "Isaac Newton, physicist") |
| >>> m.group(0) # The entire match |
| 'Isaac Newton' |
| >>> m.group(1) # The first parenthesized subgroup. |
| 'Isaac' |
| >>> m.group(2) # The second parenthesized subgroup. |
| 'Newton' |
| >>> m.group(1, 2) # Multiple arguments give us a tuple. |
| ('Isaac', 'Newton') |
| |
| If the regular expression uses the ``(?P<name>...)`` syntax, the *groupN* |
| arguments may also be strings identifying groups by their group name. If a |
| string argument is not used as a group name in the pattern, an :exc:`IndexError` |
| exception is raised. |
| |
| A moderately complicated example: |
| |
| >>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\w+)", "Malcolm Reynolds") |
| >>> m.group('first_name') |
| 'Malcolm' |
| >>> m.group('last_name') |
| 'Reynolds' |
| |
| Named groups can also be referred to by their index: |
| |
| >>> m.group(1) |
| 'Malcolm' |
| >>> m.group(2) |
| 'Reynolds' |
| |
| If a group matches multiple times, only the last match is accessible: |
| |
| >>> m = re.match(r"(..)+", "a1b2c3") # Matches 3 times. |
| >>> m.group(1) # Returns only the last match. |
| 'c3' |
| |
| |
| .. method:: match.groups(default=None) |
| |
| Return a tuple containing all the subgroups of the match, from 1 up to however |
| many groups are in the pattern. The *default* argument is used for groups that |
| did not participate in the match; it defaults to ``None``. |
| |
| For example: |
| |
| >>> m = re.match(r"(\d+)\.(\d+)", "24.1632") |
| >>> m.groups() |
| ('24', '1632') |
| |
| If we make the decimal place and everything after it optional, not all groups |
| might participate in the match. These groups will default to ``None`` unless |
| the *default* argument is given: |
| |
| >>> m = re.match(r"(\d+)\.?(\d+)?", "24") |
| >>> m.groups() # Second group defaults to None. |
| ('24', None) |
| >>> m.groups('0') # Now, the second group defaults to '0'. |
| ('24', '0') |
| |
| |
| .. method:: match.groupdict(default=None) |
| |
| Return a dictionary containing all the *named* subgroups of the match, keyed by |
| the subgroup name. The *default* argument is used for groups that did not |
| participate in the match; it defaults to ``None``. For example: |
| |
| >>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\w+)", "Malcolm Reynolds") |
| >>> m.groupdict() |
| {'first_name': 'Malcolm', 'last_name': 'Reynolds'} |
| |
| |
| .. method:: match.start([group]) |
| match.end([group]) |
| |
| Return the indices of the start and end of the substring matched by *group*; |
| *group* defaults to zero (meaning the whole matched substring). Return ``-1`` if |
| *group* exists but did not contribute to the match. For a match object *m*, and |
| a group *g* that did contribute to the match, the substring matched by group *g* |
| (equivalent to ``m.group(g)``) is :: |
| |
| m.string[m.start(g):m.end(g)] |
| |
| Note that ``m.start(group)`` will equal ``m.end(group)`` if *group* matched a |
| null string. For example, after ``m = re.search('b(c?)', 'cba')``, |
| ``m.start(0)`` is 1, ``m.end(0)`` is 2, ``m.start(1)`` and ``m.end(1)`` are both |
| 2, and ``m.start(2)`` raises an :exc:`IndexError` exception. |
| |
| An example that will remove *remove_this* from email addresses: |
| |
| >>> email = "tony@tiremove_thisger.net" |
| >>> m = re.search("remove_this", email) |
| >>> email[:m.start()] + email[m.end():] |
| 'tony@tiger.net' |
| |
| |
| .. method:: match.span([group]) |
| |
| For a match *m*, return the 2-tuple ``(m.start(group), m.end(group))``. Note |
| that if *group* did not contribute to the match, this is ``(-1, -1)``. |
| *group* defaults to zero, the entire match. |
| |
| |
| .. attribute:: match.pos |
| |
| The value of *pos* which was passed to the :meth:`~regex.search` or |
| :meth:`~regex.match` method of a :ref:`regex object <re-objects>`. This is |
| the index into the string at which the RE engine started looking for a match. |
| |
| |
| .. attribute:: match.endpos |
| |
| The value of *endpos* which was passed to the :meth:`~regex.search` or |
| :meth:`~regex.match` method of a :ref:`regex object <re-objects>`. This is |
| the index into the string beyond which the RE engine will not go. |
| |
| |
| .. attribute:: match.lastindex |
| |
| The integer index of the last matched capturing group, or ``None`` if no group |
| was matched at all. For example, the expressions ``(a)b``, ``((a)(b))``, and |
| ``((ab))`` will have ``lastindex == 1`` if applied to the string ``'ab'``, while |
| the expression ``(a)(b)`` will have ``lastindex == 2``, if applied to the same |
| string. |
| |
| |
| .. attribute:: match.lastgroup |
| |
| The name of the last matched capturing group, or ``None`` if the group didn't |
| have a name, or if no group was matched at all. |
| |
| |
| .. attribute:: match.re |
| |
| The regular expression object whose :meth:`~regex.match` or |
| :meth:`~regex.search` method produced this match instance. |
| |
| |
| .. attribute:: match.string |
| |
| The string passed to :meth:`~regex.match` or :meth:`~regex.search`. |
| |
| |
| .. _re-examples: |
| |
| Regular Expression Examples |
| --------------------------- |
| |
| |
| Checking for a Pair |
| ^^^^^^^^^^^^^^^^^^^ |
| |
| In this example, we'll use the following helper function to display match |
| objects a little more gracefully: |
| |
| .. testcode:: |
| |
| def displaymatch(match): |
| if match is None: |
| return None |
| return '<Match: %r, groups=%r>' % (match.group(), match.groups()) |
| |
| Suppose you are writing a poker program where a player's hand is represented as |
| a 5-character string with each character representing a card, "a" for ace, "k" |
| for king, "q" for queen, "j" for jack, "t" for 10, and "2" through "9" |
| representing the card with that value. |
| |
| To see if a given string is a valid hand, one could do the following: |
| |
| >>> valid = re.compile(r"^[a2-9tjqk]{5}$") |
| >>> displaymatch(valid.match("akt5q")) # Valid. |
| "<Match: 'akt5q', groups=()>" |
| >>> displaymatch(valid.match("akt5e")) # Invalid. |
| >>> displaymatch(valid.match("akt")) # Invalid. |
| >>> displaymatch(valid.match("727ak")) # Valid. |
| "<Match: '727ak', groups=()>" |
| |
| That last hand, ``"727ak"``, contained a pair, or two of the same valued cards. |
| To match this with a regular expression, one could use backreferences as such: |
| |
| >>> pair = re.compile(r".*(.).*\1") |
| >>> displaymatch(pair.match("717ak")) # Pair of 7s. |
| "<Match: '717', groups=('7',)>" |
| >>> displaymatch(pair.match("718ak")) # No pairs. |
| >>> displaymatch(pair.match("354aa")) # Pair of aces. |
| "<Match: '354aa', groups=('a',)>" |
| |
| To find out what card the pair consists of, one could use the |
| :meth:`~match.group` method of the match object in the following manner: |
| |
| .. doctest:: |
| |
| >>> pair.match("717ak").group(1) |
| '7' |
| |
| # Error because re.match() returns None, which doesn't have a group() method: |
| >>> pair.match("718ak").group(1) |
| Traceback (most recent call last): |
| File "<pyshell#23>", line 1, in <module> |
| re.match(r".*(.).*\1", "718ak").group(1) |
| AttributeError: 'NoneType' object has no attribute 'group' |
| |
| >>> pair.match("354aa").group(1) |
| 'a' |
| |
| |
| Simulating scanf() |
| ^^^^^^^^^^^^^^^^^^ |
| |
| .. index:: single: scanf() |
| |
| Python does not currently have an equivalent to :c:func:`scanf`. Regular |
| expressions are generally more powerful, though also more verbose, than |
| :c:func:`scanf` format strings. The table below offers some more-or-less |
| equivalent mappings between :c:func:`scanf` format tokens and regular |
| expressions. |
| |
| +--------------------------------+---------------------------------------------+ |
| | :c:func:`scanf` Token | Regular Expression | |
| +================================+=============================================+ |
| | ``%c`` | ``.`` | |
| +--------------------------------+---------------------------------------------+ |
| | ``%5c`` | ``.{5}`` | |
| +--------------------------------+---------------------------------------------+ |
| | ``%d`` | ``[-+]?\d+`` | |
| +--------------------------------+---------------------------------------------+ |
| | ``%e``, ``%E``, ``%f``, ``%g`` | ``[-+]?(\d+(\.\d*)?|\.\d+)([eE][-+]?\d+)?`` | |
| +--------------------------------+---------------------------------------------+ |
| | ``%i`` | ``[-+]?(0[xX][\dA-Fa-f]+|0[0-7]*|\d+)`` | |
| +--------------------------------+---------------------------------------------+ |
| | ``%o`` | ``[-+]?[0-7]+`` | |
| +--------------------------------+---------------------------------------------+ |
| | ``%s`` | ``\S+`` | |
| +--------------------------------+---------------------------------------------+ |
| | ``%u`` | ``\d+`` | |
| +--------------------------------+---------------------------------------------+ |
| | ``%x``, ``%X`` | ``[-+]?(0[xX])?[\dA-Fa-f]+`` | |
| +--------------------------------+---------------------------------------------+ |
| |
| To extract the filename and numbers from a string like :: |
| |
| /usr/sbin/sendmail - 0 errors, 4 warnings |
| |
| you would use a :c:func:`scanf` format like :: |
| |
| %s - %d errors, %d warnings |
| |
| The equivalent regular expression would be :: |
| |
| (\S+) - (\d+) errors, (\d+) warnings |
| |
| |
| .. _search-vs-match: |
| |
| search() vs. match() |
| ^^^^^^^^^^^^^^^^^^^^ |
| |
| .. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org> |
| |
| Python offers two different primitive operations based on regular expressions: |
| :func:`re.match` checks for a match only at the beginning of the string, while |
| :func:`re.search` checks for a match anywhere in the string (this is what Perl |
| does by default). |
| |
| For example:: |
| |
| >>> re.match("c", "abcdef") # No match |
| >>> re.search("c", "abcdef") # Match |
| <_sre.SRE_Match object; span=(2, 3), match='c'> |
| |
| Regular expressions beginning with ``'^'`` can be used with :func:`search` to |
| restrict the match at the beginning of the string:: |
| |
| >>> re.match("c", "abcdef") # No match |
| >>> re.search("^c", "abcdef") # No match |
| >>> re.search("^a", "abcdef") # Match |
| <_sre.SRE_Match object; span=(0, 1), match='a'> |
| |
| Note however that in :const:`MULTILINE` mode :func:`match` only matches at the |
| beginning of the string, whereas using :func:`search` with a regular expression |
| beginning with ``'^'`` will match at the beginning of each line. |
| |
| >>> re.match('X', 'A\nB\nX', re.MULTILINE) # No match |
| >>> re.search('^X', 'A\nB\nX', re.MULTILINE) # Match |
| <_sre.SRE_Match object; span=(4, 5), match='X'> |
| |
| |
| Making a Phonebook |
| ^^^^^^^^^^^^^^^^^^ |
| |
| :func:`split` splits a string into a list delimited by the passed pattern. The |
| method is invaluable for converting textual data into data structures that can be |
| easily read and modified by Python as demonstrated in the following example that |
| creates a phonebook. |
| |
| First, here is the input. Normally it may come from a file, here we are using |
| triple-quoted string syntax: |
| |
| >>> text = """Ross McFluff: 834.345.1254 155 Elm Street |
| ... |
| ... Ronald Heathmore: 892.345.3428 436 Finley Avenue |
| ... Frank Burger: 925.541.7625 662 South Dogwood Way |
| ... |
| ... |
| ... Heather Albrecht: 548.326.4584 919 Park Place""" |
| |
| The entries are separated by one or more newlines. Now we convert the string |
| into a list with each nonempty line having its own entry: |
| |
| .. doctest:: |
| :options: +NORMALIZE_WHITESPACE |
| |
| >>> entries = re.split("\n+", text) |
| >>> entries |
| ['Ross McFluff: 834.345.1254 155 Elm Street', |
| 'Ronald Heathmore: 892.345.3428 436 Finley Avenue', |
| 'Frank Burger: 925.541.7625 662 South Dogwood Way', |
| 'Heather Albrecht: 548.326.4584 919 Park Place'] |
| |
| Finally, split each entry into a list with first name, last name, telephone |
| number, and address. We use the ``maxsplit`` parameter of :func:`split` |
| because the address has spaces, our splitting pattern, in it: |
| |
| .. doctest:: |
| :options: +NORMALIZE_WHITESPACE |
| |
| >>> [re.split(":? ", entry, 3) for entry in entries] |
| [['Ross', 'McFluff', '834.345.1254', '155 Elm Street'], |
| ['Ronald', 'Heathmore', '892.345.3428', '436 Finley Avenue'], |
| ['Frank', 'Burger', '925.541.7625', '662 South Dogwood Way'], |
| ['Heather', 'Albrecht', '548.326.4584', '919 Park Place']] |
| |
| The ``:?`` pattern matches the colon after the last name, so that it does not |
| occur in the result list. With a ``maxsplit`` of ``4``, we could separate the |
| house number from the street name: |
| |
| .. doctest:: |
| :options: +NORMALIZE_WHITESPACE |
| |
| >>> [re.split(":? ", entry, 4) for entry in entries] |
| [['Ross', 'McFluff', '834.345.1254', '155', 'Elm Street'], |
| ['Ronald', 'Heathmore', '892.345.3428', '436', 'Finley Avenue'], |
| ['Frank', 'Burger', '925.541.7625', '662', 'South Dogwood Way'], |
| ['Heather', 'Albrecht', '548.326.4584', '919', 'Park Place']] |
| |
| |
| Text Munging |
| ^^^^^^^^^^^^ |
| |
| :func:`sub` replaces every occurrence of a pattern with a string or the |
| result of a function. This example demonstrates using :func:`sub` with |
| a function to "munge" text, or randomize the order of all the characters |
| in each word of a sentence except for the first and last characters:: |
| |
| >>> def repl(m): |
| ... inner_word = list(m.group(2)) |
| ... random.shuffle(inner_word) |
| ... return m.group(1) + "".join(inner_word) + m.group(3) |
| >>> text = "Professor Abdolmalek, please report your absences promptly." |
| >>> re.sub(r"(\w)(\w+)(\w)", repl, text) |
| 'Poefsrosr Aealmlobdk, pslaee reorpt your abnseces plmrptoy.' |
| >>> re.sub(r"(\w)(\w+)(\w)", repl, text) |
| 'Pofsroser Aodlambelk, plasee reoprt yuor asnebces potlmrpy.' |
| |
| |
| Finding all Adverbs |
| ^^^^^^^^^^^^^^^^^^^ |
| |
| :func:`findall` matches *all* occurrences of a pattern, not just the first |
| one as :func:`search` does. For example, if one was a writer and wanted to |
| find all of the adverbs in some text, he or she might use :func:`findall` in |
| the following manner: |
| |
| >>> text = "He was carefully disguised but captured quickly by police." |
| >>> re.findall(r"\w+ly", text) |
| ['carefully', 'quickly'] |
| |
| |
| Finding all Adverbs and their Positions |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| If one wants more information about all matches of a pattern than the matched |
| text, :func:`finditer` is useful as it provides :ref:`match objects |
| <match-objects>` instead of strings. Continuing with the previous example, if |
| one was a writer who wanted to find all of the adverbs *and their positions* in |
| some text, he or she would use :func:`finditer` in the following manner: |
| |
| >>> text = "He was carefully disguised but captured quickly by police." |
| >>> for m in re.finditer(r"\w+ly", text): |
| ... print('%02d-%02d: %s' % (m.start(), m.end(), m.group(0))) |
| 07-16: carefully |
| 40-47: quickly |
| |
| |
| Raw String Notation |
| ^^^^^^^^^^^^^^^^^^^ |
| |
| Raw string notation (``r"text"``) keeps regular expressions sane. Without it, |
| every backslash (``'\'``) in a regular expression would have to be prefixed with |
| another one to escape it. For example, the two following lines of code are |
| functionally identical: |
| |
| >>> re.match(r"\W(.)\1\W", " ff ") |
| <_sre.SRE_Match object; span=(0, 4), match=' ff '> |
| >>> re.match("\\W(.)\\1\\W", " ff ") |
| <_sre.SRE_Match object; span=(0, 4), match=' ff '> |
| |
| When one wants to match a literal backslash, it must be escaped in the regular |
| expression. With raw string notation, this means ``r"\\"``. Without raw string |
| notation, one must use ``"\\\\"``, making the following lines of code |
| functionally identical: |
| |
| >>> re.match(r"\\", r"\\") |
| <_sre.SRE_Match object; span=(0, 1), match='\\'> |
| >>> re.match("\\\\", r"\\") |
| <_sre.SRE_Match object; span=(0, 1), match='\\'> |
| |
| |
| Writing a Tokenizer |
| ^^^^^^^^^^^^^^^^^^^ |
| |
| A `tokenizer or scanner <http://en.wikipedia.org/wiki/Lexical_analysis>`_ |
| analyzes a string to categorize groups of characters. This is a useful first |
| step in writing a compiler or interpreter. |
| |
| The text categories are specified with regular expressions. The technique is |
| to combine those into a single master regular expression and to loop over |
| successive matches:: |
| |
| import collections |
| import re |
| |
| Token = collections.namedtuple('Token', ['typ', 'value', 'line', 'column']) |
| |
| def tokenize(code): |
| keywords = {'IF', 'THEN', 'ENDIF', 'FOR', 'NEXT', 'GOSUB', 'RETURN'} |
| token_specification = [ |
| ('NUMBER', r'\d+(\.\d*)?'), # Integer or decimal number |
| ('ASSIGN', r':='), # Assignment operator |
| ('END', r';'), # Statement terminator |
| ('ID', r'[A-Za-z]+'), # Identifiers |
| ('OP', r'[+\-*/]'), # Arithmetic operators |
| ('NEWLINE', r'\n'), # Line endings |
| ('SKIP', r'[ \t]+'), # Skip over spaces and tabs |
| ('MISMATCH',r'.'), # Any other character |
| ] |
| tok_regex = '|'.join('(?P<%s>%s)' % pair for pair in token_specification) |
| line_num = 1 |
| line_start = 0 |
| for mo in re.finditer(tok_regex, code): |
| kind = mo.lastgroup |
| value = mo.group(kind) |
| if kind == 'NEWLINE': |
| line_start = mo.end() |
| line_num += 1 |
| elif kind == 'SKIP': |
| pass |
| elif kind == 'MISMATCH': |
| raise RuntimeError('%r unexpected on line %d' % (value, line_num)) |
| else: |
| if kind == 'ID' and value in keywords: |
| kind = value |
| column = mo.start() - line_start |
| yield Token(kind, value, line_num, column) |
| |
| statements = ''' |
| IF quantity THEN |
| total := total + price * quantity; |
| tax := price * 0.05; |
| ENDIF; |
| ''' |
| |
| for token in tokenize(statements): |
| print(token) |
| |
| The tokenizer produces the following output:: |
| |
| Token(typ='IF', value='IF', line=2, column=4) |
| Token(typ='ID', value='quantity', line=2, column=7) |
| Token(typ='THEN', value='THEN', line=2, column=16) |
| Token(typ='ID', value='total', line=3, column=8) |
| Token(typ='ASSIGN', value=':=', line=3, column=14) |
| Token(typ='ID', value='total', line=3, column=17) |
| Token(typ='OP', value='+', line=3, column=23) |
| Token(typ='ID', value='price', line=3, column=25) |
| Token(typ='OP', value='*', line=3, column=31) |
| Token(typ='ID', value='quantity', line=3, column=33) |
| Token(typ='END', value=';', line=3, column=41) |
| Token(typ='ID', value='tax', line=4, column=8) |
| Token(typ='ASSIGN', value=':=', line=4, column=12) |
| Token(typ='ID', value='price', line=4, column=15) |
| Token(typ='OP', value='*', line=4, column=21) |
| Token(typ='NUMBER', value='0.05', line=4, column=23) |
| Token(typ='END', value=';', line=4, column=27) |
| Token(typ='ENDIF', value='ENDIF', line=5, column=4) |
| Token(typ='END', value=';', line=5, column=9) |