Karim wrote:
> Recall:
>
> >>> re.subn(r'([^\\])?"', r'\1\\"', expression)
>
> Traceback (most recent call last):
> File "<stdin>", line 1, in<module>
> File "/home/karim/build/python/install/lib/python2.7/re.py", line
> 162, in subn
> return _compile(pattern, flags).subn(repl, string, count)
> File "/home/karim/build/python/install/lib/python2.7/re.py", line
> 278, in filter
> return sre_parse.expand_template(template, match)
> File "/home/karim/build/python/install/lib/python2.7/sre_parse.py",
> line 787, in expand_template
> raise error, "unmatched group"
> sre_constants.error: unmatched group
>
>
> Found the solution: '?' needs to be inside parenthesis (saved pattern)
> because outside we don't know if the saved match argument
> will exist or not namely '\1'.
>
> >>> re.subn(r'([^\\]?)"', r'\1\\"', expression)
>
> (' \\"\\" ', 2)
>
> sed unix command is more permissive: sed 's/\([^\\]\)\?"/\1\\"/g'
> because '?' can be outside parenthesis (saved pattern but escaped for
> sed). \1 seems to not cause issue when matching is found. Perhaps it is
> created only when match occurs.
Thanks for reporting the explanation.
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