Doc/library/re.rst

: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:`match` and :func:`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 a 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(s):
        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
        ]
        tok_regex = '|'.join('(?P<%s>%s)' % pair for pair in token_specification)
        get_token = re.compile(tok_regex).match
        line = 1
        pos = line_start = 0
        mo = get_token(s)
        while mo is not None:
            typ = mo.lastgroup
            if typ == 'NEWLINE':
                line_start = pos
                line += 1
            elif typ != 'SKIP':
                val = mo.group(typ)
                if typ == 'ID' and val in keywords:
                    typ = val
                yield Token(typ, val, line, mo.start()-line_start)
            pos = mo.end()
            mo = get_token(s, pos)
        if pos != len(s):
            raise RuntimeError('Unexpected character %r on line %d' %(s[pos], line))

    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=5)
    Token(typ='ID', value='quantity', line=2, column=8)
    Token(typ='THEN', value='THEN', line=2, column=17)
    Token(typ='ID', value='total', line=3, column=9)
    Token(typ='ASSIGN', value=':=', line=3, column=15)
    Token(typ='ID', value='total', line=3, column=18)
    Token(typ='OP', value='+', line=3, column=24)
    Token(typ='ID', value='price', line=3, column=26)
    Token(typ='OP', value='*', line=3, column=32)
    Token(typ='ID', value='quantity', line=3, column=34)
    Token(typ='END', value=';', line=3, column=42)
    Token(typ='ID', value='tax', line=4, column=9)
    Token(typ='ASSIGN', value=':=', line=4, column=13)
    Token(typ='ID', value='price', line=4, column=16)
    Token(typ='OP', value='*', line=4, column=22)
    Token(typ='NUMBER', value='0.05', line=4, column=24)
    Token(typ='END', value=';', line=4, column=28)
    Token(typ='ENDIF', value='ENDIF', line=5, column=5)
    Token(typ='END', value=';', line=5, column=10)