Python Regex Unmatched Groups error with multiple patterns

Question

So I was trying to answer a question on SO when I ran into this issue. Basically a user had the following string:

Adobe.Flash.Player.14.00.125.ie

and wanted to replace it with

Adobe Flash Player 14.00.125 ie

so I used the following re.sub call to solve this issue:

re.sub("([a-zA-Z])\.([a-zA-Z0-9])",r"\1 \2",str)

I then realized that doesn't remove the dot between 125 and ie so I figured I'd try to match another pattern namely:

re.sub("([a-zA-Z])\.([a-zA-Z0-9])|([0-9])\.([a-zA-Z])",r"\1\3 \2\4",str)

When I try to run this, I get the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.6/re.py", line 151, in sub
    return _compile(pattern, 0).sub(repl, string, count)
  File "/usr/lib64/python2.6/re.py", line 278, in filter
    return sre_parse.expand_template(template, match)
  File "/usr/lib64/python2.6/sre_parse.py", line 793, in expand_template
    raise error, "unmatched group"
sre_constants.error: unmatched group

Now, I understand that it's complaining because I'm trying to replace the match with an unmatched group but is there a way around this without having to call re.sub twice?

Avinash Raj · Accepted Answer · 2014-07-23 19:44:05Z

1

Without any capturing groups,

>>> import re
>>> s = "Adobe.Flash.Player.14.00.125.ie"
>>> m = re.sub(r'\.(?=[A-Za-z])|(?<!\d)\.', r' ', s)
>>> m
'Adobe Flash Player 14.00.125 ie'

answered Jul 23, 2014 at 19:44

Avinash Raj

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4 Comments

Florin Stingaciu Over a year ago

Yeah, another answer on that question used this method. I was interested in seeing how you can reference captured groups if they don't match. +1 for an alternative.

Avinash Raj Over a year ago

if we use the logical OR operator in regex while back-referencing, then it's impossible to refer the grouped characters through back reference.

Florin Stingaciu Over a year ago

Aha. Thanks @Avinash Raj

Avinash Raj Over a year ago

The main thing in regex is, we need to solve the problem without any capturing groups(ie; through matching only). If it's not possible through matching then only we go for grouping.

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