Python regex ordering issue

Question

I have been running into a problem with python's re module.

This is the simplest version of the issue

import re

print re.findall('a|ab','aab')      # ['a', 'a']
print re.findall('ab|a','aab')      # ['a', 'ab']

I generate regexes at runtime from a text file and cannot be certain that they will be in the correct order. Is there any way around this issue?

Joran Beasley · Accepted Answer · 2012-07-09 19:00:05Z

2

sort them by size... longer first

eg:

s_regs = sorted(regexes,key=lambda x:len(x))
s_regs.reverse()
regex = '|'.join(s_regs)

answered Jul 9, 2012 at 19:00

Joran Beasley

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2 Comments

The same but slightly more efficient regex = '|'.join(sorted(regexes, key=len, reverse=True))

I would also use re.escape for every regex.