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:mod:`re` --- Regular expression operations
===========================================
.. module:: re
:synopsis: Regular expression operations.
.. moduleauthor:: Fredrik Lundh <fredrik@pythonware.com>
.. sectionauthor:: Andrew M. Kuchling <amk@amk.ca>
**Source code:** :source:`Lib/re/`
--------------
This module provides regular expression matching operations similar to
those found in Perl.
Both patterns and strings to be searched can be Unicode strings (:class:`str`)
as well as 8-bit strings (:class:`bytes`).
However, Unicode strings and 8-bit strings cannot be mixed:
that is, you cannot match a 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. Also, please note that any invalid escape sequences in Python's
usage of the backslash in string literals now generate a :exc:`SyntaxWarning`
and in the future this will become a :exc:`SyntaxError`. This behaviour
will happen even if it is a valid escape sequence for a regular expression.
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::
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 [Frie09]_,
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.
Repetition operators or quantifiers (``*``, ``+``, ``?``, ``{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:
.. index:: single: . (dot); in regular expressions
``.``
(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.
.. index:: single: ^ (caret); in regular expressions
``^``
(Caret.) Matches the start of the string, and in :const:`MULTILINE` mode also
matches immediately after each newline.
.. index:: single: $ (dollar); in regular expressions
``$``
Matches the end of the string or just before the newline at the end of the
string, and in :const:`MULTILINE` mode also matches before a newline. ``foo``
matches both 'foo' and 'foobar', while the regular expression ``foo$`` 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.
.. index:: single: * (asterisk); in regular expressions
``*``
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.
.. index:: single: + (plus); in regular expressions
``+``
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'.
.. index:: single: ? (question mark); in regular expressions
``?``
Causes the resulting RE to match 0 or 1 repetitions of the preceding RE.
``ab?`` will match either 'a' or 'ab'.
.. index::
single: *?; in regular expressions
single: +?; in regular expressions
single: ??; in regular expressions
``*?``, ``+?``, ``??``
The ``'*'``, ``'+'``, and ``'?'`` quantifiers 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 quantifier 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>'``.
.. index::
single: *+; in regular expressions
single: ++; in regular expressions
single: ?+; in regular expressions
``*+``, ``++``, ``?+``
Like the ``'*'``, ``'+'``, and ``'?'`` quantifiers, those where ``'+'`` is
appended also match as many times as possible.
However, unlike the true greedy quantifiers, these do not allow
back-tracking when the expression following it fails to match.
These are known as :dfn:`possessive` quantifiers.
For example, ``a*a`` will match ``'aaaa'`` because the ``a*`` will match
all 4 ``'a'``\ s, but, when the final ``'a'`` is encountered, the
expression is backtracked so that in the end the ``a*`` ends up matching
3 ``'a'``\ s total, and the fourth ``'a'`` is matched by the final ``'a'``.
However, when ``a*+a`` is used to match ``'aaaa'``, the ``a*+`` will
match all 4 ``'a'``, but when the final ``'a'`` fails to find any more
characters to match, the expression cannot be backtracked and will thus
fail to match.
``x*+``, ``x++`` and ``x?+`` are equivalent to ``(?>x*)``, ``(?>x+)``
and ``(?>x?)`` correspondingly.
.. versionadded:: 3.11
.. index::
single: {} (curly brackets); in regular expressions
``{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 quantifier. 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.
``{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
*without* establishing any backtracking points.
This is the possessive version of the quantifier above.
For example, on the 6-character string ``'aaaaaa'``, ``a{3,5}+aa``
attempt to match 5 ``'a'`` characters, then, requiring 2 more ``'a'``\ s,
will need more characters than available and thus fail, while
``a{3,5}aa`` will match with ``a{3,5}`` capturing 5, then 4 ``'a'``\ s
by backtracking and then the final 2 ``'a'``\ s are matched by the final
``aa`` in the pattern.
``x{m,n}+`` is equivalent to ``(?>x{m,n})``.
.. versionadded:: 3.11
.. index:: single: \ (backslash); in regular expressions
``\``
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.
.. index::
single: [] (square brackets); in regular 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'``.
.. index:: single: - (minus); in regular expressions
* 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]`` or ``[a-]``), it will match a literal ``'-'``.
* Special characters lose their special meaning inside sets. For example,
``[(+*)]`` will match any of the literal characters ``'('``, ``'+'``,
``'*'``, or ``')'``.
.. index:: single: \ (backslash); in regular expressions
* 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.
.. index:: single: ^ (caret); in regular expressions
* 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 match a right bracket, as well as left bracket, braces,
and parentheses.
.. .. index:: single: --; in regular expressions
.. .. index:: single: &&; in regular expressions
.. .. index:: single: ~~; in regular expressions
.. .. index:: single: ||; in regular expressions
* Support of nested sets and set operations as in `Unicode Technical
Standard #18`_ might be added in the future. This would change the
syntax, so to facilitate this change a :exc:`FutureWarning` will be raised
in ambiguous cases for the time being.
That includes sets starting with a literal ``'['`` or containing literal
character sequences ``'--'``, ``'&&'``, ``'~~'``, and ``'||'``. To
avoid a warning escape them with a backslash.
.. _Unicode Technical Standard #18: https://unicode.org/reports/tr18/
.. versionchanged:: 3.7
:exc:`FutureWarning` is raised if a character set contains constructs
that will change semantically in the future.
.. index:: single: | (vertical bar); in regular expressions
``|``
``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 ``[|]``.
.. index::
single: () (parentheses); in regular expressions
``(...)``
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: ``[(]``, ``[)]``.
.. index:: single: (?; in regular expressions
``(?...)``
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),
:const:`re.U` (Unicode matching), 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. Flags should be used first in the
expression string.
.. versionchanged:: 3.11
This construction can only be used at the start of the expression.
.. index:: single: (?:; in regular expressions
``(?:...)``
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.
``(?aiLmsux-imsx:...)``
(Zero or more letters from the set ``'a'``, ``'i'``, ``'L'``, ``'m'``,
``'s'``, ``'u'``, ``'x'``, optionally followed by ``'-'`` followed by
one or more letters from the ``'i'``, ``'m'``, ``'s'``, ``'x'``.)
The letters set or remove 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), :const:`re.U` (Unicode matching),
and :const:`re.X` (verbose), for the part of the expression.
(The flags are described in :ref:`contents-of-module-re`.)
The letters ``'a'``, ``'L'`` and ``'u'`` are mutually exclusive when used
as inline flags, so they can't be combined or follow ``'-'``. Instead,
when one of them appears in an inline group, it overrides the matching mode
in the enclosing group. In Unicode patterns ``(?a:...)`` switches to
ASCII-only matching, and ``(?u:...)`` switches to Unicode matching
(default). In byte pattern ``(?L:...)`` switches to locale depending
matching, and ``(?a:...)`` switches to ASCII-only matching (default).
This override is only in effect for the narrow inline group, and the
original matching mode is restored outside of the group.
.. versionadded:: 3.6
.. versionchanged:: 3.7
The letters ``'a'``, ``'L'`` and ``'u'`` also can be used in a group.
``(?>...)``
Attempts to match ``...`` as if it was a separate regular expression, and
if successful, continues to match the rest of the pattern following it.
If the subsequent pattern fails to match, the stack can only be unwound
to a point *before* the ``(?>...)`` because once exited, the expression,
known as an :dfn:`atomic group`, has thrown away all stack points within
itself.
Thus, ``(?>.*).`` would never match anything because first the ``.*``
would match all characters possible, then, having nothing left to match,
the final ``.`` would fail to match.
Since there are no stack points saved in the Atomic Group, and there is
no stack point before it, the entire expression would thus fail to match.
.. versionadded:: 3.11
.. index:: single: (?P<; in regular expressions
``(?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 in :class:`bytes` patterns they can only contain
bytes in the ASCII range. 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`` |
+---------------------------------------+----------------------------------+
.. versionchanged:: 3.12
In :class:`bytes` patterns, group *name* can only contain bytes
in the ASCII range (``b'\x00'``-``b'\x7f'``).
.. index:: single: (?P=; in regular expressions
``(?P=name)``
A backreference to a named group; it matches whatever text was matched by the
earlier group named *name*.
.. index:: single: (?#; in regular expressions
``(?#...)``
A comment; the contents of the parentheses are simply ignored.
.. index:: single: (?=; in regular expressions
``(?=...)``
Matches if ``...`` matches next, but doesn't consume any of the string. This is
called a :dfn:`lookahead assertion`. For example, ``Isaac (?=Asimov)`` will match
``'Isaac '`` only if it's followed by ``'Asimov'``.
.. index:: single: (?!; in regular expressions
``(?!...)``
Matches if ``...`` doesn't match next. This is a :dfn:`negative lookahead assertion`.
For example, ``Isaac (?!Asimov)`` will match ``'Isaac '`` only if it's *not*
followed by ``'Asimov'``.
.. index:: single: (?<=; in regular expressions
``(?<=...)``
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(r'(?<=-)\w+', 'spam-egg')
>>> m.group(0)
'egg'
.. versionchanged:: 3.5
Added support for group references of fixed length.
.. index:: single: (?<!; in regular expressions
``(?<!...)``
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.
.. _re-conditional-expression:
.. index:: single: (?(; in regular expressions
``(?(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)>|$)`` is a poor email matching pattern, which
will match with ``'<user@host.com>'`` as well as ``'user@host.com'``, but
not with ``'<user@host.com'`` nor ``'user@host.com>'``.
.. versionchanged:: 3.12
Group *id* can only contain ASCII digits.
In :class:`bytes` patterns, group *name* can only contain bytes
in the ASCII range (``b'\x00'``-``b'\x7f'``).
The special sequences consist of ``'\'`` and a character from the list below.
If the ordinary character is not an ASCII digit or an ASCII letter, then the
resulting RE will match the second character. For example, ``\$`` matches the
character ``'$'``.
.. index:: single: \ (backslash); in regular expressions
``\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.
.. index:: single: \A; in regular expressions
``\A``
Matches only at the start of the string.
.. index:: single: \b; in regular expressions
``\b``
Matches the empty string, but only at the beginning or end of a word.
A word is defined as a sequence of word characters. 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 in Unicode patterns, but
this can be changed by using the :const:`ASCII` flag. Word boundaries are
determined by the current locale if the :const:`LOCALE` flag is used.
Inside a character range, ``\b`` represents the backspace character, for
compatibility with Python's string literals.
.. index:: single: \B; in regular expressions
``\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 in Unicode
patterns are Unicode alphanumerics or the underscore, although this can
be changed by using the :const:`ASCII` flag. Word boundaries are
determined by the current locale if the :const:`LOCALE` flag is used.
.. index:: single: \d; in regular expressions
``\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.
For 8-bit (bytes) patterns:
Matches any decimal digit; this is equivalent to ``[0-9]``.
.. index:: single: \D; in regular expressions
``\D``
Matches any character which is not a decimal digit. This is
the opposite of ``\d``. If the :const:`ASCII` flag is used this
becomes the equivalent of ``[^0-9]``.
.. index:: single: \s; in regular expressions
``\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.
For 8-bit (bytes) patterns:
Matches characters considered whitespace in the ASCII character set;
this is equivalent to ``[ \t\n\r\f\v]``.
.. index:: single: \S; in regular expressions
``\S``
Matches any character which is not a 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]``.
.. index:: single: \w; in regular expressions
``\w``
For Unicode (str) patterns:
Matches Unicode word characters; this includes alphanumeric characters (as defined by :meth:`str.isalnum`)
as well as the underscore (``_``).
If the :const:`ASCII` flag is used, only ``[a-zA-Z0-9_]`` is matched.
For 8-bit (bytes) patterns:
Matches characters considered alphanumeric in the ASCII character set;
this is equivalent to ``[a-zA-Z0-9_]``. If the :const:`LOCALE` flag is
used, matches characters considered alphanumeric in the current locale
and the underscore.
.. index:: single: \W; in regular expressions
``\W``
Matches any character which is not a word character. This is
the opposite of ``\w``. If the :const:`ASCII` flag is used this
becomes the equivalent of ``[^a-zA-Z0-9_]``. If the :const:`LOCALE` flag is
used, matches characters which are neither alphanumeric in the current locale
nor the underscore.
.. index:: single: \Z; in regular expressions
``\Z``
Matches only at the end of the string.
.. index::
single: \a; in regular expressions
single: \b; in regular expressions
single: \f; in regular expressions
single: \n; in regular expressions
single: \N; in regular expressions
single: \r; in regular expressions
single: \t; in regular expressions
single: \u; in regular expressions
single: \U; in regular expressions
single: \v; in regular expressions
single: \x; in regular expressions
single: \\; in regular expressions
Most of the standard escapes supported by Python string literals are also
accepted by the regular expression parser::
\a \b \f \n
\N \r \t \u
\U \v \x \\
(Note that ``\b`` is used to represent word boundaries, and means "backspace"
only inside character classes.)
``'\u'``, ``'\U'``, and ``'\N'`` escape sequences are only recognized in Unicode
patterns. In bytes patterns they are errors. Unknown escapes of ASCII
letters are reserved for future use and treated as errors.
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.
.. versionchanged:: 3.6
Unknown escapes consisting of ``'\'`` and an ASCII letter now are errors.
.. versionchanged:: 3.8
The ``'\N{name}'`` escape sequence has been added. As in string literals,
it expands to the named Unicode character (e.g. ``'\N{EM DASH}'``).
.. _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.
Flags
^^^^^
.. versionchanged:: 3.6
Flag constants are now instances of :class:`RegexFlag`, which is a subclass of
:class:`enum.IntFlag`.
.. class:: RegexFlag
An :class:`enum.IntFlag` class containing the regex options listed below.
.. versionadded:: 3.11 - added to ``__all__``
.. 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.
Corresponds to the inline flag ``(?a)``.
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.
No corresponding inline flag.
.. data:: I
IGNORECASE
Perform case-insensitive matching; expressions like ``[A-Z]`` will also
match lowercase letters. Full Unicode matching (such as ``Ü`` matching
``ü``) also works unless the :const:`re.ASCII` flag is used to disable
non-ASCII matches. The current locale does not change the effect of this
flag unless the :const:`re.LOCALE` flag is also used.
Corresponds to the inline flag ``(?i)``.
Note that when the Unicode patterns ``[a-z]`` or ``[A-Z]`` are used in
combination with the :const:`IGNORECASE` flag, they will match the 52 ASCII
letters and 4 additional non-ASCII letters: 'İ' (U+0130, Latin capital
letter I with dot above), 'ı' (U+0131, Latin small letter dotless i),
'ſ' (U+017F, Latin small letter long s) and 'K' (U+212A, Kelvin sign).
If the :const:`ASCII` flag is used, only letters 'a' to 'z'
and 'A' to 'Z' are matched.
.. data:: L
LOCALE
Make ``\w``, ``\W``, ``\b``, ``\B`` and case-insensitive matching
dependent on the current locale. This flag can be used only with bytes
patterns. The use of this flag is discouraged as the locale mechanism
is very unreliable, it only handles one "culture" at a time, and it only
works with 8-bit locales. Unicode matching is already enabled by default
in Python 3 for Unicode (str) patterns, and it is able to handle different
locales/languages.
Corresponds to the inline flag ``(?L)``.
.. versionchanged:: 3.6
:const:`re.LOCALE` can be used only with bytes patterns and is
not compatible with :const:`re.ASCII`.
.. versionchanged:: 3.7
Compiled regular expression objects with the :const:`re.LOCALE` flag no
longer depend on the locale at compile time. Only the locale at
matching time affects the result of matching.
.. 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.
Corresponds to the inline flag ``(?m)``.
.. data:: NOFLAG
Indicates no flag being applied, the value is ``0``. This flag may be used
as a default value for a function keyword argument or as a base value that
will be conditionally ORed with other flags. Example of use as a default
value::
def myfunc(text, flag=re.NOFLAG):
return re.match(text, flag)
.. versionadded:: 3.11
.. data:: S
DOTALL
Make the ``'.'`` special character match any character at all, including a
newline; without this flag, ``'.'`` will match anything *except* a newline.
Corresponds to the inline flag ``(?s)``.
.. data:: X
VERBOSE
.. index:: single: # (hash); in regular expressions
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<...>``. For example, ``(? :``
and ``* ?`` are not allowed.
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*")
Corresponds to the inline flag ``(?x)``.
Functions
^^^^^^^^^
.. function:: compile(pattern, flags=0)
Compile a regular expression pattern into a :ref:`regular expression object
<re-objects>`, which can be used for matching using its
:func:`~Pattern.match`, :func:`~Pattern.search` and other 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.
.. function:: search(pattern, string, flags=0)
Scan through *string* looking for the first location where the regular expression
*pattern* produces a match, and return a corresponding :ref:`match object
<match-objects>`. Return ``None`` if no position in the string matches the
pattern; note that this is different from finding a zero-length match at some
point in the string.
.. function:: match(pattern, string, flags=0)
If zero or more characters at the beginning of *string* match the regular
expression *pattern*, return a corresponding :ref:`match object
<match-objects>`. Return ``None`` if the string does not match the pattern;
note that this is different from a zero-length match.
Note that even in :const:`MULTILINE` mode, :func:`re.match` will only match
at the beginning of the string and not at the beginning of each line.
If you want to locate a match anywhere in *string*, use :func:`search`
instead (see also :ref:`search-vs-match`).
.. function:: fullmatch(pattern, string, flags=0)
If the whole *string* matches the regular expression *pattern*, return a
corresponding :ref:`match object <match-objects>`. Return ``None`` if the
string does not match the pattern; note that this is different from a
zero-length match.
.. versionadded:: 3.4
.. function:: split(pattern, string, maxsplit=0, flags=0)
Split *string* by the occurrences of *pattern*. If capturing parentheses are
used in *pattern*, then the text of all groups in the pattern are also returned
as part of the resulting list. If *maxsplit* is nonzero, at most *maxsplit*
splits occur, and the remainder of the string is returned as the final element
of the list. ::
>>> re.split(r'\W+', 'Words, words, words.')
['Words', 'words', 'words', '']
>>> re.split(r'(\W+)', 'Words, words, words.')
['Words', ', ', 'words', ', ', 'words', '.', '']
>>> re.split(r'\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(r'(\W+)', '...words, words...')
['', '...', 'words', ', ', 'words', '...', '']
That way, separator components are always found at the same relative
indices within the result list.
Empty matches for the pattern split the string only when not adjacent
to a previous empty match.
>>> re.split(r'\b', 'Words, words, words.')
['', 'Words', ', ', 'words', ', ', 'words', '.']
>>> re.split(r'\W*', '...words...')
['', '', 'w', 'o', 'r', 'd', 's', '', '']
>>> re.split(r'(\W*)', '...words...')
['', '...', '', '', 'w', '', 'o', '', 'r', '', 'd', '', 's', '...', '', '', '']
.. versionchanged:: 3.1
Added the optional flags argument.
.. versionchanged:: 3.7
Added support of splitting on a pattern that could match an empty string.
.. function:: findall(pattern, string, flags=0)
Return all non-overlapping matches of *pattern* in *string*, as a list of
strings or tuples. The *string* is scanned left-to-right, and matches
are returned in the order found. Empty matches are included in the result.
The result depends on the number of capturing groups in the pattern.
If there are no groups, return a list of strings matching the whole
pattern. If there is exactly one group, return a list of strings
matching that group. If multiple groups are present, return a list
of tuples of strings matching the groups. Non-capturing groups do not
affect the form of the result.
>>> re.findall(r'\bf[a-z]*', 'which foot or hand fell fastest')
['foot', 'fell', 'fastest']
>>> re.findall(r'(\w+)=(\d+)', 'set width=20 and height=10')
[('width', '20'), ('height', '10')]
.. versionchanged:: 3.7
Non-empty matches can now start just after a previous empty 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.
.. versionchanged:: 3.7
Non-empty matches can now start just after a previous empty 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 of ASCII letters are reserved for future use and
treated as errors. Other unknown escapes such as ``\&`` 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 :ref:`match object <match-objects>`
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 a :ref:`pattern object <re-objects>`.
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 empty match, so ``sub('x*', '-', 'abxd')`` returns
``'-a-b--d-'``.
.. index:: single: \g; in regular expressions
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