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.

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 :class:`RegexObject` methods.  The functions are
shortcuts that don't require you to compile a regex object first, but miss some
fine-tuning parameters.

.. seealso::

   The third-party `regex <https://pypi.org/project/regex/>`_ module,
   which has an API compatible with the standard library :mod:`re` module,
   but offers additional functionality and a more thorough Unicode support.


.. _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 the ``\number`` notation, e.g., ``'\x00'``.

Repetition qualifiers (``*``, ``+``, ``?``, ``{m,n}``, etc) cannot be
directly nested. This avoids ambiguity with the non-greedy modifier suffix
``?``, and with other modifiers in other implementations. To apply a second
repetition to an inner repetition, parentheses may be used. For example,
the expression ``(?:a{6})*`` matches any multiple of six ``'a'`` characters.


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 ``<a> b <c>``, it will match the entire
   string, and not just ``<a>``.  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 the RE ``<.*?>`` will match
   only ``<a>``.

``{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:`LOCALE` or  :const:`UNICODE` 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.

``(?iLmsux)``
   (One or more letters from the set ``'i'``, ``'L'``, ``'m'``, ``'s'``,
   ``'u'``, ``'x'``.)  The group matches the empty string; the letters
   set the corresponding flags: :const:`re.I` (ignore case),
   :const:`re.L` (locale dependent), :const:`re.M` (multi-line),
   :const:`re.S` (dot matches all), :const:`re.U` (Unicode dependent),
   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.  Group
   references are not supported even if they match strings of some fixed length.
   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 and shouldn't contain group references.
   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'``.

   .. versionadded:: 2.4

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 alphanumeric or underscore characters, so the end of a
   word is indicated by whitespace or a non-alphanumeric, non-underscore 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, so the precise set of characters deemed to be alphanumeric
   depends on the values of the ``UNICODE`` and ``LOCALE`` flags.
   For example, ``r'\bfoo\b'`` matches ``'foo'``, ``'foo.'``, ``'(foo)'``,
   ``'bar foo baz'`` but not ``'foobar'`` or ``'foo3'``.
   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 is also subject to the settings
   of ``LOCALE`` and ``UNICODE``.

``\d``
   When the :const:`UNICODE` flag is not specified, matches any decimal digit; this
   is equivalent to the set ``[0-9]``.  With :const:`UNICODE`, it will match
   whatever is classified as a decimal digit in the Unicode character properties
   database.

``\D``
   When the :const:`UNICODE` flag is not specified, matches any non-digit
   character; this is equivalent to the set  ``[^0-9]``.  With :const:`UNICODE`, it
   will match  anything other than character marked as digits in the Unicode
   character  properties database.

``\s``
   When the :const:`UNICODE` flag is not specified, it matches any whitespace
   character, this is equivalent to the set ``[ \t\n\r\f\v]``. The
   :const:`LOCALE` flag has no extra effect on matching of the space.
   If :const:`UNICODE` is set, this will match the characters ``[ \t\n\r\f\v]``
   plus whatever is classified as space in the Unicode character properties
   database.

``\S``
   When the :const:`UNICODE` flag is not specified, matches any non-whitespace
   character; this is equivalent to the set ``[^ \t\n\r\f\v]`` The
   :const:`LOCALE` flag has no extra effect on non-whitespace match.  If
   :const:`UNICODE` is set, then any character not marked as space in the
   Unicode character properties database is matched.


``\w``
   When the :const:`LOCALE` and :const:`UNICODE` flags are not specified, matches
   any alphanumeric character and the underscore; this is equivalent to the set
   ``[a-zA-Z0-9_]``.  With :const:`LOCALE`, it will match the set ``[0-9_]`` plus
   whatever characters are defined as alphanumeric for the current locale.  If
   :const:`UNICODE` is set, this will match the characters ``[0-9_]`` plus whatever
   is classified as alphanumeric in the Unicode character properties database.

``\W``
   When the :const:`LOCALE` and :const:`UNICODE` flags are not specified, matches
   any non-alphanumeric character; this is equivalent to the set ``[^a-zA-Z0-9_]``.
   With :const:`LOCALE`, it will match any character not in the set ``[0-9_]``, and
   not defined as alphanumeric for the current locale. If :const:`UNICODE` is set,
   this will match anything other than ``[0-9_]`` plus characters classified as
   not alphanumeric in the Unicode character properties database.

``\Z``
   Matches only at the end of the string.

If both :const:`LOCALE` and :const:`UNICODE` flags are included for a
particular sequence, then :const:`LOCALE` flag takes effect first followed by
the :const:`UNICODE`.

Most of the standard escapes supported by Python string literals are also
accepted by the regular expression parser::

   \a      \b      \f      \n
   \r      \t      \v      \x
   \\

(Note that ``\b`` is used to represent word boundaries, and means "backspace"
only inside character classes.)

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.

.. 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.



.. _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:`~RegexObject.match` and
   :func:`~RegexObject.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.match`, :func:`re.search` or :func:`re.compile` are cached, so
      programs that use only a few regular expressions at a time needn't worry
      about compiling regular expressions.


.. 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.  To
   get this effect on non-ASCII Unicode characters such as ``ü`` and ``Ü``,
   add the :const:`UNICODE` flag.


.. data:: L
          LOCALE

   Make ``\w``, ``\W``, ``\b``, ``\B``, ``\s`` and ``\S`` dependent on the
   current locale.


.. 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:: U
          UNICODE

   Make the ``\w``, ``\W``, ``\b``, ``\B``, ``\d``, ``\D``, ``\s`` and ``\S``
   sequences dependent on the Unicode character properties database. Also
   enables non-ASCII matching for :const:`IGNORECASE`.

   .. versionadded:: 2.0


.. data:: X
          VERBOSE

   This flag allows you to write regular expressions that look nicer and are
   more readable by allowing you to visually separate logical sections of the
   pattern and add comments. Whitespace within the pattern is ignored, except
   when in a character class, or when preceded by an unescaped backslash,
   or within tokens like ``*?``, ``(?:`` or ``(?P<...>``.
   When a line contains a ``#`` that is not in a character class and is not
   preceded by an unescaped backslash, all characters from the leftmost such
   ``#`` through the end of the line are ignored.

   This 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 :class:`MatchObject`
   instance. 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 :class:`MatchObject` instance.
   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:: 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.  (Incompatibility note: in the original Python 1.5 release,
   *maxsplit* was ignored.  This has been fixed in later releases.)

      >>> 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 (e.g., if there's one capturing group
   in the separator, the 0th, the 2nd and so forth).

   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:: 2.7
      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.

   .. note::

      Due to the limitation of the current implementation the character
      following an empty match is not included in a next match, so
      ``findall(r'^|\w+', 'two words')`` returns ``['', 'wo', 'words']``
      (note missed "t").  This is changed in Python 3.7.

   .. versionadded:: 1.5.2

   .. versionchanged:: 2.4
      Added the optional flags argument.


.. function:: finditer(pattern, string, flags=0)

   Return an :term:`iterator` yielding :class:`MatchObject` instances 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.  See also the note about :func:`findall`.

   .. versionadded:: 2.2

   .. versionchanged:: 2.4
      Added the optional flags argument.


.. 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:: 2.7
      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:: 2.7
      Added the optional flags argument.


.. function:: escape(pattern)

   Escape all the characters in *pattern* except ASCII letters and numbers.
   This is useful if you want to match an arbitrary literal string that may
   have regular expression metacharacters in it.  For example::

      >>> print re.escape('python.exe')
      python\.exe

      >>> legal_chars = string.ascii_lowercase + string.digits + "!#$%&'*+-.^_`|~:"
      >>> print '[%s]+' % re.escape(legal_chars)
      [abcdefghijklmnopqrstuvwxyz0123456789\!\#\$\%\&\'\*\+\-\.\^\_\`\|\~\:]+

      >>> operators = ['+', '-', '*', '/', '**']
      >>> print '|'.join(map(re.escape, sorted(operators, reverse=True)))
      \/|\-|\+|\*\*|\*


.. 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
--------------------------

.. class:: RegexObject

   The :class:`RegexObject` class supports the following methods and attributes:

   .. method:: RegexObject.search(string[, pos[, endpos]])

      Scan through *string* looking for a location where this regular expression
      produces a match, and return a corresponding :class:`MatchObject` instance.
      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 at ...>
      >>> pattern.search("dog", 1)  # No match; search doesn't include the "d"


   .. method:: RegexObject.match(string[, pos[, endpos]])

      If zero or more characters at the *beginning* of *string* match this regular
      expression, return a corresponding :class:`MatchObject` instance.  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:`~RegexObject.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 at ...>

      If you want to locate a match anywhere in *string*, use
      :meth:`~RegexObject.search` instead (see also :ref:`search-vs-match`).


   .. method:: RegexObject.split(string, maxsplit=0)

      Identical to the :func:`split` function, using the compiled pattern.


   .. method:: RegexObject.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:: RegexObject.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:: RegexObject.sub(repl, string, count=0)

      Identical to the :func:`sub` function, using the compiled pattern.


   .. method:: RegexObject.subn(repl, string, count=0)

      Identical to the :func:`subn` function, using the compiled pattern.


   .. attribute:: RegexObject.flags

      The regex matching flags.  This is a combination of the flags given to
      :func:`.compile` and any ``(?...)`` inline flags in the pattern.


   .. attribute:: RegexObject.groups

      The number of capturing groups in the pattern.


   .. attribute:: RegexObject.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:: RegexObject.pattern

      The pattern string from which the RE object was compiled.


.. _match-objects:

Match Objects
-------------

.. class:: MatchObject

   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:: MatchObject.expand(template)

      Return the string obtained by doing backslash substitution on the template
      string *template*, as done by the :meth:`~RegexObject.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:: MatchObject.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:: MatchObject.groups([default])

      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``.  (Incompatibility
      note: in the original Python 1.5 release, if the tuple was one element long, a
      string would be returned instead.  In later versions (from 1.5.1 on), a
      singleton tuple is returned in such cases.)

      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:: MatchObject.groupdict([default])

      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:: MatchObject.start([group])
               MatchObject.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:: MatchObject.span([group])

      For :class:`MatchObject` *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:: MatchObject.pos

      The value of *pos* which was passed to the :meth:`~RegexObject.search` or
      :meth:`~RegexObject.match` method of the :class:`RegexObject`.  This is the
      index into the string at which the RE engine started looking for a match.


   .. attribute:: MatchObject.endpos

      The value of *endpos* which was passed to the :meth:`~RegexObject.search` or
      :meth:`~RegexObject.match` method of the :class:`RegexObject`.  This is the
      index into the string beyond which the RE engine will not go.


   .. attribute:: MatchObject.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:: MatchObject.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:: MatchObject.re

      The regular expression object whose :meth:`~RegexObject.match` or
      :meth:`~RegexObject.search` method produced this :class:`MatchObject`
      instance.


   .. attribute:: MatchObject.string

      The string passed to :meth:`~RegexObject.match` or
      :meth:`~RegexObject.search`.


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:`~MatchObject.group` method of :class:`MatchObject` 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 at ...>

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 at ...>

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 at ...>


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 a writer wanted to
find all of the adverbs in some text, they 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 instances of
:class:`MatchObject` instead of strings.  Continuing with the previous example,
if a writer wanted to find all of the adverbs *and their positions*
in some text, they 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 at ...>
   >>> re.match("\\W(.)\\1\\W", " ff ")
   <_sre.SRE_Match object at ...>

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 at ...>
   >>> re.match("\\\\", r"\\")
   <_sre.SRE_Match object at ...>