maestromusica maestromusica - 9 months ago 52
Python Question

Regex for a third-person verb

I'm trying to create a regex that matches a third person form of a verb created using the following rule:

If the verb ends in e not preceded by i,o,s,x,z,ch,sh, add s.

So I'm looking for a regex matching a word consisting of some letters, then not i,o,s,x,z,ch,sh, and then "es". I tried this:


According to regex101 it matches "likes", "hates" etc. However, it does not match "bathes", why doesn't it?

Answer Source

You may use


See the regex demo

Since Python re does not support variable length alternatives in a lookbehind, you need to split the conditions into two lookbehinds here.

Pattern details:

  • \b - a leading word boundary
  • (?=\w*(?<![iosxz])(?<![cs]h)es\b) - a positive lookahead requiring a sequence of:
    • \w* - 0+ word chars
    • (?<![iosxz]) - there must not be i, o, s, x, z chars right before the current location and...
    • (?<![cs]h) - no ch or sh right before the current location...
    • es - followed with es...
    • \b - at the end of the word
  • \w* - zero or more (maybe + is better here to match 1 or more) word chars.

See Python demo:

import re
r = re.compile(r'\b(?=\w*(?<![iosxz])(?<![cs]h)es\b)\w*')
s = 'it matches "likes", "hates" etc. However, it does not match "bathes", why doesn\'t it?'
print(re.findall(r, s))