Python — check if named capture group exists

March 1, 2022

I’m wondering what the proper way to test if a named capture group exists. Specifically, I have a function that takes a compiled regex as a parameter. The regex may or may not have a specific named group, and the named group may or may not be present in a string being passed in:

some_regex = re.compile("^foo(?P<idx>[0-9]*)?$")
other_regex = re.compile("^bar$")

def some_func(regex, string):
    m=regex.match(regex,string)
    if m.group("idx"):     # get *** IndexError: no such group here...
        print(f"index found and is {idx}")
    print(f"no index found")

some_func(other_regex,"bar")

I’d like to test if the group exists without using try — as this would short circuit the rest of the function, which I would still need to run if the named group was not found

>Solution :

You can check the groupdict of the match object:

import re
some_regex = re.compile("^foo(?P<idx>[0-9]*)?$")

match = some_regex.match('foo11')
print(True) if match and 'idx' in match.groupdict() else print(False) # True
match = some_regex.match('bar11')
print(True) if match and 'idx' in match.groupdict() else print(False) # False