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

def some_func(regex, group_name):
   return group_name in regex.groupindex

print(some_func(some_regex,"bar"))
print(some_func(some_regex,"idx"))
print(some_func(other_regex,"bar"))
print(some_func(other_regex,"idx"))