Backport Python 3.4's regular expression "fullmatch()" to Python 2

Question

Python 3.4 introduced the new regex method re.fullmatch(pattern, string, flags=0).

Has anyone back-ported this new method to older Python versions?

Tim Pietzcker · Accepted Answer · 2015-05-13T11:35:02.590

22

To make sure that the entire string matches, you need to use the \Z end-of-string anchor:

def fullmatch(regex, string, flags=0):
    """Emulate python-3.4 re.fullmatch()."""
    return re.match("(?:" + regex + r")\Z", string, flags=flags)

The \A anchor is not necessary since re.match() already anchors the match to the start of the string.

edited May 13 '15 at 11:35

answered May 13 '15 at 10:56

Tim Pietzcker

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Can also use `$` to match until end-of-line. – gaborous Jan 01 '17 at 14:42
1

@gaborous: That might backfire since `regex` could contain a `(?m)` modifier, and then less than the entire string could match. – Tim Pietzcker Feb 21 '18 at 08:35
Does the `r")\Z"` part need to be a raw string literal, or maybe this is just a habit of yours? Neither Python 2 nor 3 treat `\Z` in any special way, so I think this is not mandatory, but who knows... – Freddie Chopin Mar 16 '18 at 21:39

score 2 · Answer 2 · edited May 23 '17 at 12:26

Here it is my backporting, which has at least one problem (thanks tim-pietzcker) but does not require recompiling regexes:

import re

def fullmatch(regex, string, flags=0):
    """Emulate python-3.4 re.fullmatch()."""
    m = re.match(regex, string, flags=flags)
    if m and m.span()[1] == len(string):
        return m

And here are some test-cases proving the above emulation-function.

def compare_expansion(regex, s, template):
    m1 = re.fullmatch(regex, s)
    s1 = m1.expand(template) if m1 else '<NO-MATCH>'
    m2 = fullmatch(regex, s)
    s2 = m2.expand(template) if m2 else '<NO-MATCH>'
    if s1 != s2:
        raise AssertionError("\n  PY-3: '%s' \n  PY-2: '%s' " % (s1, s2))

compare_expansion('.*', 'foo', r'A')
compare_expansion('(.*)', 'foo', r'A_\1')
compare_expansion('(.*)', 'foo', r'A_\g<0>')

compare_expansion('a.*', 'afoo&', r'A')
compare_expansion('a(\w*)', 'afoo&', r'A_\1')
compare_expansion('a(\w*)', 'afoo&', r'A_\g<0>')

## But this fails!
compare_expansion(".*?", "Hello", '\g<0>')
AssertionError: 
  PY-3: 'A_Hello' 
  PY-2: '<NO-MATCH>'

This fails with lazy quantifiers: `re.fullmatch(".*?", "Hello")` returns `None` where it should return a match. — Tim Pietzcker, May 13 '15 at 10:53

Backport Python 3.4's regular expression "fullmatch()" to Python 2

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