New submission from James Davis <davis...@vt.edu>:
I have two regexes: /(a|ab)*?b/ and /(ab|a)*?b/.
If I re.search the string "ab" for these regexes, I get inconsistent behavior.
Specifically, /(a|ab)*?b/ matches with capture "a", while /(ab|a)*?b/ matches
with an empty capture group.
I am not actually sure which behavior is correct.
Interpretation 1: The (ab|a) clause matches the a, satisfying the (ab|a)*?
once, and the engine proceeds to the b and completes. The capture group ends up
containing "a".
Interpretation 2: The (ab|a) clause matches the a. Since the clause is marked
with *, the engine repeats the attempt and finds nothing the second time. It
proceeds to the b and completes. Because the second match attempt on (ab|a)
found nothing, the capture group ends up empty.
The behavior depends on both the order of (ab|a) vs. (a|ab), and the use of the
non-greedy quantifier.
I cannot see why changing the order of the alternation should have this effect.
The change in behavior occurs in the built-in "re" module but not in the
competing "regex" module.
The behavior is consistent in both Python 2.7 and Python 3.5. I have not tested
other versions.
I have included the confusing-regex-behavior.py file for troubleshooting.
Below is the behavior for matches on these and many variants.
I find the following lines the most striking:
Regex pattern matched? matched string captured
content
-------------------- -------------------- --------------------
--------------------
(ab|a)*?b True ab
('',)
(ab|a)+?b True ab
('',)
(ab|a){0,}?b True ab
('',)
(ab|a){0,2}?b True ab
('',)
(ab|a){0,1}?b True ab
('a',)
(ab|a)*b True ab
('a',)
(ab|a)+b True ab
('a',)
(a|ab)*?b True ab
('a',)
(a|ab)+?b True ab
('a',)
(08:58:48) jamie@woody ~ $ python3 /tmp/confusing-regex-behavior.py
Behavior from re
Regex pattern matched? matched string captured
content
-------------------- -------------------- --------------------
--------------------
(ab|a)*?b True ab
('',)
(ab|a)+?b True ab
('',)
(ab|a){0,}?b True ab
('',)
(ab|a){0,2}?b True ab
('',)
(ab|a)?b True ab
('a',)
(ab|a)??b True ab
('a',)
(ab|a)b True ab
('a',)
(ab|a){0,1}?b True ab
('a',)
(ab|a)*b True ab
('a',)
(ab|a)+b True ab
('a',)
(a|ab)*b True ab
('a',)
(a|ab)+b True ab
('a',)
(a|ab)*?b True ab
('a',)
(a|ab)+?b True ab
('a',)
(a|ab)*?b True ab
('a',)
(a|ab)*?b True ab
('a',)
(a|ab)*?b True ab
('a',)
(a|ab)*?b True ab
('a',)
(bb|a)*?b True ab
('a',)
((?:ab|a)*?)b True ab
('a',)
((?:a|ab)*?)b True ab
('a',)
Behavior from regex
Regex pattern matched? matched string captured
content
-------------------- -------------------- --------------------
--------------------
(ab|a)*?b True ab
('a',)
(ab|a)+?b True ab
('a',)
(ab|a){0,}?b True ab
('a',)
(ab|a){0,2}?b True ab
('a',)
(ab|a)?b True ab
('a',)
(ab|a)??b True ab
('a',)
(ab|a)b True ab
('a',)
(ab|a){0,1}?b True ab
('a',)
(ab|a)*b True ab
('a',)
(ab|a)+b True ab
('a',)
(a|ab)*b True ab
('a',)
(a|ab)+b True ab
('a',)
(a|ab)*?b True ab
('a',)
(a|ab)+?b True ab
('a',)
(a|ab)*?b True ab
('a',)
(a|ab)*?b True ab
('a',)
(a|ab)*?b True ab
('a',)
(a|ab)*?b True ab
('a',)
(bb|a)*?b True ab
('a',)
((?:ab|a)*?)b True ab
('a',)
((?:a|ab)*?)b True ab
('a',)
----------
components: Regular Expressions
files: confusing-regex-behavior.py
messages: 334560
nosy: davisjam, ezio.melotti, mrabarnett
priority: normal
severity: normal
status: open
title: Capture behavior depends on the order of an alternation
type: behavior
versions: Python 2.7, Python 3.5
Added file: https://bugs.python.org/file48085/confusing-regex-behavior.py
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue35859>
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