Message 29112 - Python tracker (original) (raw)

Using sre.sub[n], an "unmatched group" error can occur.

The test I used is this pattern:

sre.sub("foo(?:b(ar)|baz)","\1","foobaz")

This will cause the following backtrace to occur:

Traceback (most recent call last): File "", line 1, in ? File "lib/python2.4/sre.py", line 142, in sub return _compile(pattern, 0).sub(repl, string, count) File "lib/python2.4/sre.py", line 260, in filter return sre_parse.expand_template(template, match) File "lib/python2.4/sre_parse.py", line 782, in expand_template raise error, "unmatched group" sre_constants.error: unmatched group

Python Version 2.4.3, Mac OS X (behaviour has been verified on Windows 2.4.3 as well).

This behaviour, while by design, is unwanted because this type of matching usually requests that a blank match be returned (i.e. the example should return '')

The example that I was trying resembles the following:

sre.sub("User: (?:Registered User #(\d+)|Guest)","%USERID|\1%",data)

The intended behaviour is that the function returns "" when the user is a guest and the user number if the user is a registered member.

However, when this function encounters a Guest, it raises an exception and terminates, which is not what is wanted.

Perl and other regex engines behave as I have described, substituting empty strings for unmatched groups. The code fix is relatively simple, and would really help out for these types of things.