Note

For testing and comparison with the current 're' module the new implementation is in the form of a module called 'regex'.

Please note that certain aspects of this module have changed from those of previous versions to make it conform better to the 're' module.

Building for 64-bits

If the source files are built for a 64-bit target then the string positions will also be 64-bit. (The 're' module appears to limit string positions to 32 bits, even on a 64-bit build.)

Flags

There are 2 kinds of flag: scoped and global. Scoped flags can apply to only part of a pattern and can be turned on or off; global flags apply to the entire pattern and can only be turned on.

The scoped flags are: IGNORECASE, MULTILINE, DOTALL, VERBOSE, WORD.

The global flags are: ASCII, LOCALE, NEW, REVERSE, UNICODE.

The NEW flag turns on the new behaviour of this module, which can differ from that of the 're' module, such as splitting on zero-width matches, inline flags affecting only what follows, and being able to turn inline flags off.

Note: The ZEROWIDTH flag which was in previous versions of this module has been removed. The NEW flag should be used instead.

Notes on named capture groups

All capture groups have a group number, starting from 1.

Groups with the same group name will have the same group number, and groups with a different group name will have a different group number.

The same group name can be used on different branches of an alternation because they are mutually exclusive, eg. (?P<foo>first)|(?P<foo>second). They will, of course, have the same group number.

Group numbers will be reused, where possible, across different branches of a branch reset, eg. (?|(first)|(second)) has only group 1. If capture groups have different group names then they will, of course, have different group numbers, eg. (?|(?P<foo>first)|(?P<bar>second)) has group 1 ("foo") and group 2 ("bar").

Unicode

This module supports Unicode 6.0.0.

Additional features

• regex.escape (issue #2650)

  regex.escape has an additional keyword parameter special_only. When True, only 'special' regex characters, such as '?', are escaped.

  Examples:

  >>> regex.escape("foo!?")
  'foo\\!\\?'
  >>> regex.escape("foo!?", special_only=True)
  'foo!\\?'
  

• Repeated captures (issue #7132)

  A match object has additional methods which return information on all the successful matches of a repeated capture group. These methods are:

  matchobject.captures([group1, ...])

  Returns a tuple of the strings matched in a group or groups. Compare with matchobject.group([group1, ...]).

  matchobject.starts([group])

  Returns a tuple of the start positions. Compare with matchobject.start([group]).

  matchobject.ends([group])

  Returns a tuple of the end positions. Compare with matchobject.end([group]).

  matchobject.spans([group])

  Returns a tuple of the spans. Compare with matchobject.span([group]).

  Examples:

  >>> m = regex.search(r"(\w{3})+", "123456789")
  >>> m.group(1)
  '789'
  >>> m.captures(1)
  ('123', '456', '789')
  >>> m.start(1)
  6
  >>> m.starts(1)
  (0, 3, 6)
  >>> m.end(1)
  9
  >>> m.ends(1)
  (3, 6, 9)
  >>> m.span(1)
  (6, 9)
  >>> m.spans(1)
  ((0, 3), (3, 6), (6, 9))
  

• Atomic grouping (issue #433030)

  (?>...)

  If the following pattern subsequently fails, then the subpattern as a whole will fail.

• Possessive quantifiers.

  (?:...)?+ ; (?:...)*+ ; (?:...)++ ; (?:...){min,max}+

  The subpattern is matched up to 'max' times. If the following pattern subsequently fails, then all of the repeated subpatterns will fail as a whole. For example, (?:...)++ is equivalent to (?>(?:...)+).

• Scoped flags (issue #433028)

  (?flags-flags:...)

  The flags will apply only to the subpattern. Flags can be turned on or off.

• Inline flags (issue #433024, issue #433027)

  (?flags-flags)

  If the NEW flag is turned on then the flags will apply to the end of the group or pattern and can be turned on or off. If the NEW flag isn't turned on then the flags will be global and can't be turned off.

• Repeated repeats (issue #2537)

  A regex like ((x|y+)*)* will be accepted and will work correctly, but should complete more quickly.

• Definition of 'word' character (issue #1693050)

  The definition of a 'word' character has been expanded for Unicode. This applies to \w, \W, \b and \B.

• Groups in lookahead and lookbehind (issue #814253)

  Groups and group references are permitted in both lookahead and lookbehind.

• Variable-length lookbehind

  A lookbehind can match a variable-length string.

• Correct handling of charset with ignore case flag (issue #3511)

  Ranges within charsets are handled correctly when the ignore-case flag is turned on.

• Unmatched group in replacement (issue #1519638)

  An unmatched group is treated as an empty string in a replacement template.

• 'Pathological' patterns (issue #1566086, issue #1662581, issue #1448325, issue #1721518, issue #1297193)

  'Pathological' patterns should complete more quickly.

• Flags argument for regex.split, regex.sub and regex.subn (issue #3482)

  regex.split, regex.sub and regex.subn support a 'flags' argument.

• Pos and endpos arguments for regex.sub and regex.subn

  regex.sub and regex.subn support 'pos' and 'endpos' arguments.

• 'Overlapped' argument for regex.findall and regex.finditer

  regex.findall and regex.finditer support an 'overlapped' flag which permits overlapped matches.

• Unicode escapes (issue #3665)

  The Unicode escapes \uxxxx and \Uxxxxxxxx are supported.

• Large patterns (issue #1160)

  Patterns can be much larger.

• Zero-width match with regex.finditer (issue #1647489)

  regex.finditer behaves correctly when it splits at a zero-width match.

• Zero-width split with regex.split (issue #3262)

  regex.split can split at a zero-width match if the NEW flag is turned on. When the flag is turned off the current behaviour is unchanged because the BDFL thinks that some existing software might depend on it.

• Splititer

  regex.splititer has been added. It's a generator equivalent of regex.split.

• Subscripting for groups

  A match object accepts access to the captured groups via subscripting and slicing:

  >>> m = regex.search(r"(?P<before>.*?)(?P<num>\d+)(?P<after>.*)", "pqr123stu")
  >>> print m["before"]
  pqr
  >>> print m["num"]
  123
  >>> print m["after"]
  stu
  >>> print len(m)
  4
  >>> print m[:]
  ('pqr123stu', 'pqr', '123', 'stu')
  

• Named groups

  Groups can be named with (?<name>...) as well as the current (?P<name>...).

• Group references

  Groups can be referenced within a pattern with \g<name>. This also allows there to be more than 99 groups.

• Named characters

  \N{name}

  Named characters are supported.

• Unicode codepoint properties, blocks and scripts

  \p{name} ; \P{name}

  Unicode properties, blocks and scripts are supported. \p{name} matches a character which has property 'name' and \P{name} matches a character which doesn't have property 'name'.

  In order to avoid ambiguity, block names should start with In and script names should start with Is. If a name lacks such a prefix and it could be a block or a script, script will take priority, for example:

  1. InBasicLatin or BasicLatin, the 'BasicLatin' block.
  2. IsLatin or Latin, the 'Latin' script.
  3. InCyrillic, the 'Cyrillic' block.
  4. IsCyrillic or Cyrillic, the 'Cyrillic' script.

• Posix character classes

  [[:alpha:]]

  Posix character classes are supported.

• Search anchor

  \G

  A search anchor has been added. It matches at the position where each search started/continued and can be used for contiguous matches or in negative variable-length lookbehinds to limit how far back the lookbehind goes:

  >>> regex.findall(r"\w{2}", "abcd ef")
  ['ab', 'cd', 'ef']
  >>> regex.findall(r"\G\w{2}", "abcd ef")
  ['ab', 'cd']
  

  1. The search starts at position 0 and matches 2 letters 'ab'.
  2. The search continues at position 2 and matches 2 letters 'cd'.
  3. The search continues at position 4 and fails to match any letters.
  4. The anchor stops the search start position from being advanced, so there are no more results.

• Reverse searching

  Searches can now work backwards:

  >>> regex.findall(r".", "abc")
  ['a', 'b', 'c']
  >>> regex.findall(r"(?r).", "abc")
  ['c', 'b', 'a']
  

  Note: the result of a reverse search is not necessarily the reverse of a forward search:

  >>> regex.findall(r"..", "abcde")
  ['ab', 'cd']
  >>> regex.findall(r"(?r)..", "abcde")
  ['de', 'bc']
  

• Multithreading

  The regex module releases the GIL when matching on instances of the built-in (immutable) string classes, enabling other Python threads to run concurrently.

• Matching a single grapheme

  \X

  The grapheme matcher is supported. It's equivalent to \P{M}\p{M}*.

• Branch reset

  (?|...|...)

  Capture group numbers will be reused across the alternatives.

• Default Unicode word boundary

  The WORD flag changes the definition of a 'word boundary' to that of a default Unicode word boundary. This applies to \b and \B.

  Please note: I'm unsure whether I've understood the specification correctly, so if you're using this feature I'd be interested in any feedback.