Velocity Reviews

Velocity Reviews (
-   Python (
-   -   Re: python regex "negative lookahead assertions" problems (

MRAB 11-22-2009 04:32 PM

Re: python regex "negative lookahead assertions" problems
Tim Chase wrote:
>>>>> 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.

> This first finds everything (".*") and then asserts that "warning"
> doesn't follow it, which is correct in your example. You may have to
> assert that "warning" doesn't exist at every point along the way:
> re.match(r'(?:(?!warning).)*',line)
> which will match up-to-but-not-including the "warning" text. If you
> don't want it at all, you'd have to also anchor the far end
> re.match(r'^(?:(?!warning).)*$',line)
> but in the 2nd case I'd just as soon invert the test:
> if 'warning' not in line:
> do_stuff()

The trick is to think what positive lookahead you'd need if you wanted
check whether 'warning' is present:


and then negate it:


giving you:

re.match(r'(?!.*warning)', line)

All times are GMT. The time now is 05:07 PM.

Powered by vBulletin®. Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO ©2010, Crawlability, Inc.