# Revision history [back]

To some extent, this might be a flaw in python's interface. There's an ambiguity to what matches.group is supposed to do:

matches.group(i)


should have the same effect as

matches.groups()[i]


or as

matches.groupdict()[i]


depending on the nature of i. As it happens, the C code decides on whether i passes PyInt_Check (i.e., really is an integer), so if i is Integer(1) then i is interpreted as a key into explicitly named groups. Unfortunately, the rules are a little strict for what the names of named groups can be, so i will never be valid as such:

sage: matches = re.match("(?:foo)(?P<1>bar)(?P<2>sws)?",ST)
error: bad character in group name


It's documented that group names must follow the same rules as valid python identifiers. Valid use is:

sage: matches = re.match("(?:foo)(?P<a>bar)(?P<b>sws)?",ST)
sage: matches.group('a')
'bar'


so the problem is a muddled interface: matches.group(i) must choose whether to try to interpret i as an integer or as a string. There is of course no good answer if i is neither and indeed, the code hardly tries anything.

In the last example, matches.group(i) should work for any i that compares equal to a or b and hashes consistently with that.

To some extent, this might be a flaw in python's interface. There's an ambiguity to what matches.group is supposed to do:

matches.group(i)


should have the same effect as

matches.groups()[i]


or as

matches.groupdict()[i]


depending on the nature of i. As it happens, the C code decides on whether i passes PyInt_Check (i.e., really is an integer), so if i is Integer(1) then i is interpreted as a key into explicitly named groups. Unfortunately, the rules are a little strict for what the names of named groups can be, so i if i is an Integer, it will never be valid as such:

sage: matches = re.match("(?:foo)(?P<1>bar)(?P<2>sws)?",ST)
error: bad character in group name


It's documented that group names must follow the same rules as valid python identifiers. Valid use is:

sage: matches = re.match("(?:foo)(?P<a>bar)(?P<b>sws)?",ST)
sage: matches.group('a')
'bar'


so the problem is a muddled interface: matches.group(i) must choose whether to try to interpret i as an integer or as a string. There is of course no good answer if i is neither and indeed, the code hardly tries anything.

In the last example, matches.group(i) should work for any i that compares equal to a or b and hashes consistently with that.