[New-bugs-announce] [issue17257] re module shows unexpected non-greedy behavior when using groups

Hendrik Lemelson report at bugs.python.org
Wed Feb 20 18:55:44 CET 2013

New submission from Hendrik Lemelson:

When using the Python 2.7.3 re module, it shows a strange behavior upon the use of quantifiers together with groups:

>>> re.search('(a*)', 'caaaat').groups()
>>> re.search('(a+)', 'caaaat').groups()
>>> re.search('(a{0,5})', 'caaaat').groups()
>>> re.search('(a{1,5})', 'caaaat').groups()

Whenever a quantifier is used that allows also zero occurrences, the quantifier loses its greedy behavior. This in my eyes is a defect in the re module. In the following there is another example with nested groups where the quantifier for the outer group even prevents the inner groups to match:

>>> re.search('(a(b*)a)', 'caabbaat').groups()
('aa', '')
>>> re.search('(a(b+)a)', 'caabbaat').groups()
('abba', 'bb')
>>> re.search('(a(b*)a){0,1}', 'caabbaat').groups()
(None, None)
>>> re.search('(a(b+)a){0,1}', 'caabbaat').groups()
(None, None)

It would be great if you could manage to fix this.
Thank you in advance.

Hendrik Lemelson

components: Regular Expressions
messages: 182535
nosy: Hendrik.Lemelson, ezio.melotti, mrabarnett, pitrou
priority: normal
severity: normal
status: open
title: re module shows unexpected non-greedy behavior when using groups
type: behavior
versions: Python 2.7

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