Python regex substitution: separate backreference from digit

Question

In a regex replacement pattern, a backreference looks like \1. If you want to include a digit after that backreference, this will fail because the digit is considered to be part of the backreference number:

# replace all twin digits by zeroes, but retain white space in between
re.sub(r"\d(\s*)\d", r"0\10", "0 1")
>>> sre_constants.error: invalid group reference

Substitution pattern r"0\1 0" would work fine but in the failing example back-reference \1 is interpreted as \10.

How can the digit '0' be separated from the back-reference \1 that precedes it?

@segfolt He asked, and answered his own question - perfectly legitimate. — Inbar Rose, May 29 '13 at 09:44
@segfolt florisla probably wanted to put a question and put the solution to it himself. — Jerry, May 29 '13 at 09:44
Ah ok I'm quite new in SO so I didn't know it was possible to do such a thing — aldeb, May 29 '13 at 09:45
I found the solution while I was typing out the question. So I used the 'Answer your own queston' checkbox. — florisla, May 29 '13 at 09:48

score 19 · Answer 1 · answered May 29 '13 at 10:01

19

You can use \g<1>, as mentioned in the docs.

answered May 29 '13 at 10:01

Janne Karila

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Typical case of not reading the docs thoroughly enough, thanks for that. – florisla May 29 '13 at 13:07

florisla · Accepted Answer · 2013-05-31T09:16:12.740

12

Instead of using a backreference with a sequence number (\1), you can use named groups and the problem is solved:

# replace all twin digits by zeroes, but retain whitespace in between
re.sub(r"\d(?P<whitespace>\s*)\d", r"0\g<whitespace>0", "0 1")
>>> '0 0'

Turns out this trick is in fact described in the documentation of re.sub.

edited May 31 '13 at 09:16

answered May 29 '13 at 09:42

florisla

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Don't forget to accept your own answer :) (I think there's a time limit though, I'm not 100% sure) – TerryA May 29 '13 at 09:52
True, there's a 2-day time limit. – florisla May 29 '13 at 13:04

Python regex substitution: separate backreference from digit

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