> From: John Bond <li...@asd-group.com> > You might wonder why something that can match no input > text, doesn't return an infinite number of those matches at > every possible position, but they would be overlapping, and > findall explicitly says matches have to be non-overlapping.
That scrabbed my itches, though the notion of overlapping empty strings is quite interesting in itself. Obviously we have to assume there is one and only one empty string between two consecutive characters. Now I slightly modified my regex, and it suddenly looks self-explanatory: >>> re.findall('((.a.)+)', 'Mary has a lamb') [('Mar', 'Mar'), ('has a lam', 'lam')] >>> re.findall('((.a.)*)', 'Mary has a lamb') [('Mar', 'Mar'), ('', ''), ('', ''), ('has a lam', 'lam'), ('', ''), ('', '')] BUT, but. 1. I expected findall to find matches of the whole regex '(.a.)+', not just the subgroup (.a.) from >>> re.findall('(.a.)+', 'Mary has a lamb') Thus it is probably a misunderstanding/bug?? 2. Here is an statement from the documentation on non-capturing groups: see http://docs.python.org/dev/howto/regex.html "Except for the fact that you can’t retrieve the contents of what the group matched, a non-capturing group behaves exactly the same as a capturing group; " Thus, I'm again confused, despite of your previous explanation. This might be a better explanation: when a subgroup is repeated, it only captures the last repetition. 3. It would be convenient to have '(*...)' for non-capturing groups -- but of course, that's only a remote suggestion. 4. By reason of greediness of '*', and the concept of non-overlapping, it should go like this for re.findall('((.a.)*)', 'Mary has a lamb') step 1: Match 'Mar' + '' (gready!) step 2: skip 'y' step 3: Match '' step 4: skip ' ' step 5: Match ''+'has'+' a '+'lam'+'' (greedy!) step 7: skip 'b' step 8: Match '' So there should be 4 matches in total: 'Mar', '', 'has a lam', '' Also, if a repeated subgroup only captures the last repetition, the repeated subgroup (.a.)* should always be ''. Yet the execution in Python results in 6 matches. Here is the documentation of re.findall: ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ findall(pattern, string, flags=0) Return a list of all non-overlapping matches in the string. If one or more groups are present in the pattern, return a list of groups; this will be a list of tuples if the pattern has more than one group. Empty matches are included in the result. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Thus from >>> re.findall('(.a.)*', 'Mary has a lamb') I should get this result [('',), ('',), ('',), ('',)] Finally, The name findall implies all matches should be returned, whether there are subgroups in the pattern or not. It might be best to return all the match objects (like a re.match call) instead of the matched strings. Then there is no need to return tuples of subgroups. Even if tuples of subgroups were to be returned, group(0) must also be included in the returned tuple. Regards, Yingjie -- http://mail.python.org/mailman/listinfo/python-list