Why is the regular expression returning an error in python?

Question

Am trying the following regular expression in python but it returns an error

import re
...

#read a line from a file to variable line
# loking for the pattern 'WORD' in the line ...

m=re.search('(?<=[WORD])\w+',str(line))
m.group(0)

i get the following error:

AttributeError: 'NoneType' object has no attribute 'group'

Answer 1

This is happening because the regular expression wasn't matched. Therefore m is None and of course you can't access group[0]. You need to first test that the search was successful, before trying to access group members.

Answer 2

Two issues:

1. your pattern does not match, therefore m is set to None, and None has no group attribute.

2. I believe you meant either:

   m= re.search(r"(?<=WORD)\w+", str(line))
   

   as entered, or

   m= re.search(r"(?P<WORD>\w+)", str(line))
   

   The former matches "abc" in "WORDabc def"; the latter matches "abc" in "abc def" and the match object will have a .group("WORD") containing "abc". (Using r"" strings is generally a good idea when specifying regular expressions.)