Bugs item #1202493, was opened at 2005-05-15 23:59
Message generated for change (Comment added) made by birkenfeld
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Category: Regular Expressions
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Submitted By: Skip Montanaro (montanaro)
Assigned to: Gustavo Niemeyer (niemeyer)
Summary: RE parser too loose with {m,n} construct
Initial Comment:
This seems wrong to me:
>>> re.match("(UNIX{})", "UNIX{}").groups()
('UNIX',)
With no numbers or commas, "{}" should not be considered
special in the pattern. The docs identify three numeric
repetition possibilities: {m}, {m,} and {m,n}. There's no
description of {} meaning anything. Either the docs should
say {} implies {1,1}, {} should have no special meaning, or
an exception should be raised during compilation of the
regular expression.
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>Comment By: Reinhold Birkenfeld (birkenfeld)
Date: 2005-06-01 23:32
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Okay. Attaching patch which does that.
BTW, these things are currently allowed too (treated as
literals):
"{"
"{x"
"{x}"
"{x,y}"
"{1,x}"
etc.
The patch changes that, too.
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Comment By: Raymond Hettinger (rhettinger)
Date: 2005-06-01 23:07
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I prefer Skip's third option, raising an exception during
compilation. This is an re syntax error. Treat it the same
way that we handle similar situations with regular Python:
>>> a[]
SyntaxError: invalid syntax
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Comment By: Reinhold Birkenfeld (birkenfeld)
Date: 2005-06-01 22:30
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So, should a {} raise an error, or warn like in Ruby?
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Comment By: Raymond Hettinger (rhettinger)
Date: 2005-06-01 22:25
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IMO, the simplest rule is that braces always be considered
special. This accommodates future extensions, simplifies
the re compiler, and makes it easier to know what needs to
be escaped.
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Comment By: Reinhold Birkenfeld (birkenfeld)
Date: 2005-06-01 18:54
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It's interesting what other RE implementations do with this
ambiguity:
Perl treats {} as literal in REs, as Skip proposes.
Ruby does, too, but issues a warning about } being unescaped.
GNU (e)grep v2.5.1 allows a bare {} only if it is at the
start of a RE, but matches it literally then.
GNU sed v4.1.4 does never allow it.
GNU awk v3.1.4 is gracious and acts like Perl.
Attached is a patch that fixes this behaviour in the
appearing "common sense".
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