[Python-Dev] re performance

Sven R. Kunze srkunze at mail.de
Thu Jan 26 16:13:11 EST 2017

Hi folks,

I recently refreshed regular expressions theoretical basics *indulging 
in reminiscences* So, I read https://swtch.com/~rsc/regexp/regexp1.html

However, reaching the chart in the lower third of the article, I saw 
Python 2.4 measured against a naive Thompson matching implementation. 
And I was surprised about how bad it performed compared to an 
unoptimized version of an older than dirt algorithm.

So, I decided to give it a try with Python2.7 and Python3.5. Eh, voilà, 
100% cpu and no results so far. Nothing has changed at all since 2007.

 >>> import re
 >>> re.match(r'a?'*30 + r'a'*30, 'a'*30)
.... (still waiting)

Quoting from the article: " Some might argue that this test is unfair to 
the backtracking implementations, since it focuses on an uncommon corner 
case. This argument misses the point: given a choice between an 
implementation with a predictable, consistent, fast running time on all 
inputs or one that usually runs quickly but can take years of CPU time 
(or more) on some inputs, the decision should be easy."

Victor, as the head of Python performance department, and Matthew, as 
the maintainer of the new regex module, what is your stance on this 
particular issue?

 From my perspective, I can say, that regular expressions might worth 
optimizing especially for web applications (url matching usually uses 
regexes) but also for other applications where I've seen many tight 
loops using regexes as well. So, I am probing interest on this topic here.


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