re.rst

:mod:`re` -- simple regular expressions

.. module:: re
   :synopsis: regular expressions

|see_cpython_module| :mod:`python:re`.

This module implements regular expression operations. Regular expression syntax supported is a subset of CPython re module (and actually is a subset of POSIX extended regular expressions).

Supported operators and special sequences are:

.

Match any character.

[...]

Match set of characters. Individual characters and ranges are supported, including negated sets (e.g. [^a-c]).

^

Match the start of the string.

$

Match the end of the string.

?

Match zero or one of the previous sub-pattern.

*

Match zero or more of the previous sub-pattern.

+

Match one or more of the previous sub-pattern.

??

Non-greedy version of ?, match zero or one, with the preference for zero.

*?

Non-greedy version of *, match zero or more, with the preference for the shortest match.

+?

Non-greedy version of +, match one or more, with the preference for the shortest match.

|

Match either the left-hand side or the right-hand side sub-patterns of this operator.

(...)

Grouping. Each group is capturing (a substring it captures can be accessed with match.group() method).

\d

Matches digit. Equivalent to [0-9].

\D

Matches non-digit. Equivalent to [^0-9].

\s

Matches whitespace. Equivalent to [ \t-\r].

\S

Matches non-whitespace. Equivalent to [^ \t-\r].

\w

Matches "word characters" (ASCII only). Equivalent to [A-Za-z0-9_].

\W

Matches non "word characters" (ASCII only). Equivalent to [^A-Za-z0-9_].

\

Escape character. Any other character following the backslash, except for those listed above, is taken literally. For example, \* is equivalent to literal * (not treated as the * operator). Note that \r, \n, etc. are not handled specially, and will be equivalent to literal letters r, n, etc. Due to this, it's not recommended to use raw Python strings (r"") for regular expressions. For example, r"\r\n" when used as the regular expression is equivalent to "rn". To match CR character followed by LF, use "\r\n".

NOT SUPPORTED:

• counted repetitions ({m,n})
• named groups ((?P<name>...))
• non-capturing groups ((?:...))
• more advanced assertions (\b, \B)
• special character escapes like \r, \n - use Python's own escaping instead
• etc.

Example:

import re

# As re doesn't support escapes itself, use of r"" strings is not
# recommended.
regex = re.compile("[\r\n]")

regex.split("line1\rline2\nline3\r\n")

# Result:
# ['line1', 'line2', 'line3', '', '']

Functions

.. function:: compile(regex_str, [flags])

   Compile regular expression, return `regex <regex>` object.

.. function:: match(regex_str, string)

   Compile *regex_str* and match against *string*. Match always happens
   from starting position in a string.

.. function:: search(regex_str, string)

   Compile *regex_str* and search it in a *string*. Unlike `match`, this will search
   string for first position which matches regex (which still may be
   0 if regex is anchored).

.. function:: sub(regex_str, replace, string, count=0, flags=0, /)

   Compile *regex_str* and search for it in *string*, replacing all matches
   with *replace*, and returning the new string.

   *replace* can be a string or a function.  If it is a string then escape
   sequences of the form ``\<number>`` and ``\g<number>`` can be used to
   expand to the corresponding group (or an empty string for unmatched groups).
   If *replace* is a function then it must take a single argument (the match)
   and should return a replacement string.

   If *count* is specified and non-zero then substitution will stop after
   this many substitutions are made.  The *flags* argument is ignored.

   Note: availability of this function depends on :term:`MicroPython port`.

.. data:: DEBUG

   Flag value, display debug information about compiled expression.
   (Availability depends on :term:`MicroPython port`.)

Regex objects

Compiled regular expression. Instances of this class are created using re.compile().

.. method:: regex.match(string)
            regex.search(string)
            regex.sub(replace, string, count=0, flags=0, /)

   Similar to the module-level functions :meth:`match`, :meth:`search`
   and :meth:`sub`.
   Using methods is (much) more efficient if the same regex is applied to
   multiple strings.

.. method:: regex.split(string, max_split=-1, /)

   Split a *string* using regex. If *max_split* is given, it specifies
   maximum number of splits to perform. Returns list of strings (there
   may be up to *max_split+1* elements if it's specified).

Match objects

Match objects as returned by match() and search() methods, and passed to the replacement function in sub().

.. method:: match.group(index)

   Return matching (sub)string. *index* is 0 for entire match,
   1 and above for each capturing group. Only numeric groups are supported.

.. method:: match.groups()

   Return a tuple containing all the substrings of the groups of the match.

   Note: availability of this method depends on :term:`MicroPython port`.

.. method:: match.start([index])
            match.end([index])

   Return the index in the original string of the start or end of the
   substring group that was matched.  *index* defaults to the entire
   group, otherwise it will select a group.

   Note: availability of these methods depends on :term:`MicroPython port`.

.. method:: match.span([index])

   Returns the 2-tuple ``(match.start(index), match.end(index))``.

   Note: availability of this method depends on :term:`MicroPython port`.