Python regexp (sre_constants.error: unmatched group)

Question

I checked all the related post on Stackoverflow, but none of the answers helped. I have the following items in a list:

topics\Utmutatok\Uzemeltetoi_Utmutato.dita
topics\_Reuse\Definitions\FRP_CUST_PROD_properties.xml
topics\_Reuse\Definitions\FRP_properties.xml
topics\_Reuse\Definitions\FR_Definitions.dita

I use the following lines in my python file:

kifejezes5 = re.sub(r'^(?!(?:topics/_Reuse(.*?)|^$)$|$).*$', r'@@@@@.\1', kifejezes4)

The expressions work perfectly in Notepad++, but not in my script.

When I run my script I get the error mentioned in the title. Is there any possible workaround here?

You have a capturing group inside a negative lookahead. The group will never be populated, otherwise, there would be no match. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Dec 29, 2016 at 17:28
Please explain what output you need to get, say, for topics\_Reuse\Definitions\FRP_CUST_PROD_properties.xml? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Dec 29, 2016 at 17:36
the output should be: @@@@@topics\_Reuse\Definitions\FRP_CUST_PROD_properties.xml — user2666249
– user2666249, Commented Dec 29, 2016 at 17:38
So, re.sub(r'^(?!(?:topics/_Reuse.*)?$).*$', r'@@@@@\g<0>', s)? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Dec 29, 2016 at 18:12

Wiktor Stribiżew · Accepted Answer · 2016-12-29 19:25:09Z

1

The group is unmatched because the capturing group is used inside a negative lookahead. If there is a match, the group is never populated.

Instead, use

re.sub(r'^(?!(?:topics/_Reuse.*)?$).*$', r'@@@@@\g<0>', s)

See the online Python demo:

import re
s = 'topics\_Reuse\Definitions\FRP_CUST_PROD_properties.xml'
res = re.sub(r'^(?!(?:topics/_Reuse.*)?$).*$', r'@@@@@\g<0>', s)
print(res)
# => @@@@@topics\_Reuse\Definitions\FRP_CUST_PROD_properties.xml

The pattern matches:

^ - start of string
(?!(?:topics/_Reuse.*)?$) - not followed with topics/_Reuse and then any 0+ chars up to the end of string OR just the end of string (the string cannot be empty)
.*$ - any 0+ chars (other than line break chars) up to the end of string.

The replacement pattern contains a \g<0> backreference to the whole match.

answered Dec 29, 2016 at 19:25

Wiktor Stribiżew

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1 Comment

user2666249 Over a year ago

Thanks for the clear explanation! I misunderstood the backreferencing.... Now it's OK!

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