Regex - optional text followed by greedily matched repeating text

Please help me fix my regex :).

Summary: How can I make a repeating group (tags) match greedily even if it means a preceding optional group (a label) is empty.

For some reason, my regex is not acting as desired:

code:     re.match("^foo(-.*?)?((?:-(?:a|b))*)$", "foo-a-b").groups()  output:   ('-a', '-b')  expected: ('', '-a-b') # since "-a" and "-b" are both tags that should be greedily matched by the last pattern  

Examples of expected behavior:

Input	Expected	Current Output	Notes
foo-a-b	('', '-a-b')	('-a', '-b')	everything after "foo" is a tag, so label should be empty
foo-b-a	('', '-b-a')	('-b', '-a')	everything after "foo" is a tag, so label should be empty
foo-c-a-b	('-c', '-a-b')	as expected	has both a label and a tag
foo-a-b-c	('-a-b-c', '')	as expected	everything is a label because tags can only be at the end

My real-life problem has a much more complicated definition of tags, labels, and "foo", but the issue is reproducible even with this smaller contrived example.

Thanks in advance!

https://stackoverflow.com/questions/66937068/regex-optional-text-followed-by-greedily-matched-repeating-text April 04, 2021 at 08:36AM