Anomalous behaviour when compiling regular expressions?

Harvey.Thomas at informa.com Harvey.Thomas at informa.com
Mon Mar 13 11:32:33 CET 2006


>>> import re
>>> r = re.compile('(a|b*)+')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "c:\python24\lib\sre.py", line 180, in compile
    return _compile(pattern, flags)
  File "c:\python24\lib\sre.py", line 227, in _compile
    raise error, v # invalid expression
sre_constants.error: nothing to repeat

but

>>> r = re.compile('(a|b*c*)+')
>>> r.match('def').group()
''

Why is there a difference in behaviour between the two cases. Surely the
two cases are equivalent to:

>>> r = re.compile('(a|b)*')
>>> r.match('def').group()
''

and 

>>> r = re.compile('(a|b|c)*')
>>> r.match('def').group()
''

Harvey

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