python regex "negative lookahead assertions" problems

Helmut Jarausch jarausch at
Sun Nov 22 16:05:47 CET 2009

On 11/22/09 14:58, Jelle Smet wrote:
> Hi List,
> I'm trying to match lines in python using the re module.
> The end goal is to have a regex which enables me to skip lines which have ok and warning in it.
> But for some reason I can't get negative lookaheads working, the way it's explained in "".
> Consider this example:
> Python 2.6.4 (r264:75706, Nov  2 2009, 14:38:03)
> [GCC 4.4.1] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>>>> import re
>>>> line='2009-11-22 12:15:441  lmqkjsfmlqshvquhsudfhqf qlsfh qsduidfhqlsiufh qlsiuf qldsfhqlsifhqlius dfh warning qlsfj lqshf lqsuhf lqksjfhqisudfh qiusdfhq iusfh'
>>>> re.match('.*(?!warning)',line)
> <_sre.SRE_Match object at 0xb75b1598>
> I would expect that this would NOT match as it's a negative lookahead and warning is in the string.

'.*' eats all of line. Now, when at end of line, there is no 'warning' anymore, so it matches.
What are you trying to achieve?

If you just want to single out lines with 'ok' or warning in it, why not just
if'(ok|warning)') : call_skip


Helmut Jarausch

Lehrstuhl fuer Numerische Mathematik
RWTH - Aachen University
D 52056 Aachen, Germany

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