Search is to long with regex like ^(.+|dontmatch)*$

[vstinner]

BPO	1297193
Nosy	@pitrou, @vstinner, @orivej
Files	bug2.py: Test file (bug2.py) bug.c: Test file in C language

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2008-09-09.11:35:35.865>
created_at = <Date 2005-09-21.03:09:18.000>
labels = ['expert-regex']
title = 'Search is to long with regex like ^(.+|dontmatch)*$'
updated_at = <Date 2008-09-09.11:35:35.863>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2008-09-09.11:35:35.863>
actor = 'pitrou'
assignee = 'niemeyer'
closed = True
closed_date = <Date 2008-09-09.11:35:35.865>
closer = 'pitrou'
components = ['Regular Expressions']
creation = <Date 2005-09-21.03:09:18.000>
creator = 'vstinner'
dependencies = []
files = ['1785', '1786']
hgrepos = []
issue_num = 1297193
keywords = []
message_count = 6.0
messages = ['26345', '26346', '26347', '26348', '72846', '72848']
nosy_count = 5.0
nosy_names = ['niemeyer', 'pitrou', 'vstinner', 'rsc', 'orivej']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1297193'
versions = ['Python 2.4']

[vstinner]

Hi,

I don't know if it's a bug or a feature, but Python
looks very slow to process the regex ^(.+|D)*A$ for
string like "xxxxxxB" (add x to run python slower).

I found this bug when a try to match strings like "abc"
or "a\"b" or "\"abc" or "" with the regex:

>\"((?:[^\\"]+|\\\")*)\"<<.

I know that ".+" construction in not ... optimal, but
Python should optimize regex like (.+|something).

Haypo

[vstinner]

Logged In: YES
user_id=365388

I try the same regex with Linux Regex library, and it looks
fast (but, can I compare C with Python ?) and not "buggy".

See bug.c file, and type gcc -o bug bug.c && time ./bug (I
get 0.000s for user ...).

Victor

[vstinner]

Logged In: YES
user_id=365388

The "bug" only occurs when the regex doesn't match, else
it's very fast to match the string.

Haypo

[vstinner]

Logged In: YES
user_id=365388

First fix: (a+|b)* can be replaced by (a|b). Other
optimizations : (a|b+) and (a+|b+) can be replaced by (a|b)*.

For the case (a*|b)+ ... Wait, I'm thinking on the problem
:-) Hum, looks like (?:(a|b)+)?. Im' not sure.

This optimizatons should be done in compile() function.

Haypo

[vstinner]

It's not a bug, it's a feature. I wrote a library
(http://hachoir.org/wiki/hachoir-regex) to "optimize" regex, so you
can close this issue.

[pitrou]

Fair enough :)