Anomalous behaviour when compiling regular expressions?

>>> 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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