For testing and comparison with the current 're' module the new implementation is in the form of a module called 'regex'.
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.
The global flags are: ASCII, LOCALE, REVERSE, UNICODE, ZEROWIDTH.
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. (?<foo>first)|(?<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. (?|(?<foo>first)|(?<bar>second)) has group 1 ("foo") and group 2 ("bar").
Atomic grouping (issue #433030)
(?>...)
If the following pattern subsequently fails, then the subpattern as a whole will fail.
(?:...)?+ ; (?:...)*+ ; (?:...)++ ; (?:...){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 (#433024, #433027)
(?flags-flags)
The flags will apply to the end of the group or pattern. Flags can be turned on or off.
Repeated repeats (#2537)
A regex like ((x|y+)*)* will be accepted and will work correctly, but should complete more quickly.
Definition of 'word' character (#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 (#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 (#3511)
Ranges within charsets are handled correctly when the ignore-case flag is turned on.
Unmatched group in replacement (#1519638)
An unmatched group is treated as an empty string in a replacement template.
'Pathological' patterns (#1566086, #1662581, #1448325, #1721518, #1297193)
'Pathological' patterns should complete more quickly.
Flags argument for regex.split, regex.sub and regex.subn (#3482)
regex.split, regex.sub and regex.subn support a 'flags' argument.
'Overlapped' argument for regex.findall and regex.finditer
regex.findall and regex.finditer support an 'overlapped' flag which permits overlapped matches.
Unicode escapes (#3665)
The Unicode escapes \uxxxx and \Uxxxxxxxx are supported.
Large patterns (#1160)
Patterns can be much larger.
Zero-width match with regex.finditer (#1647489)
regex.finditer behaves correctly when it splits at a zero-width match.
Zero-width split with regex.split (#3262)
regex.split can split at a zero-width match if the zero-width 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"(?<before>.*?)(?<num>\\d+)(?<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
Named 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:
- InBasicLatin or BasicLatin, the 'BasicLatin' block.
- IsLatin or Latin, the 'Latin' script.
- InCyrillic, the 'Cyrillic' block.
- 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']
- The search starts at position 0 and matches 2 letters 'ab'.
- The search continues at position 2 and matches 2 letters 'cd'.
- The search continues at position 4 and fails to match any letters.
- 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 now releases the GIL when matching, 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.