Get error when try to use `\1` which get from regex `(\d)?` in Python

Question

Example code:

#!/usr/bin/env python
import re

print re.sub(r'a+(\d)?', r'\1', "aaaa3")
print re.sub(r'a+(\d)?', r'\1', "aaaa") # error!

The second print statement gives me an error:

3
Traceback (most recent call last):
  File "./bbb.py", line 5, in <module>
    print re.sub(r'a+(\d)?', r'\1', "aaaa")
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/re.py", line 155, in sub
    return _compile(pattern, flags).sub(repl, string, count)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/re.py", line 291, in filter
    return sre_parse.expand_template(template, match)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sre_parse.py", line 831, in expand_template
    raise error, "unmatched group"
sre_constants.error: unmatched group

How can I deal with this capturing variable with a probable quantifier 0 without an error?

Note (\d)? here can be another complicated regex, not just an easy as \d, that why I bring my quantifier ? out out (..).

In Python, you cannot back-reference failed (unmatched) capturing groups. But you can in JavaScript :) — Wiktor Stribiżew, Sep 08 '15 at 17:46
I try this easy code on https://regex101.com/#python, it's work but on my terminal, it doesn't work, then I'm a bit confused. Any way thank you for your responses. — fronthem, Sep 08 '15 at 17:50
@stribizhev your information couple with a note about Javascript should be helpful, please answer below. — fronthem, Sep 08 '15 at 17:52
I guess the currently accepted answer explains it all, no need flooding. — Wiktor Stribiżew, Sep 08 '15 at 19:33

score 2 · Accepted Answer · edited Sep 09 '15 at 04:28

2

In Python you can do this to get an empty back-reference of an optional group:

>>> print re.sub(r'a+(\d?)', r'\1', "aaaa")

>>> print re.sub(r'a+(\d?)', r'\1', "aaaa123")
123

i.e. use (\d?) instead of (\d)?

Python regex engine unlike many other regex engines doesn't populate back-references when corresponding capturing group fails to match a pattern.

edited Sep 09 '15 at 04:28

nhahtdh

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answered Sep 08 '15 at 17:49

anubhava

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Get error when try to use `\1` which get from regex `(\d)?` in Python

1 Answers1