Question

Is there a way to determine how many capture groups there are in a given regular expression?

I would like to be able to do the follwing:

def groups(regexp, s):     ''' Returns the first result of re.findall, or an empty default      >>> groups(r'(\d)(\d)(\d)', '123')     ('1', '2', '3')     >>> groups(r'(\d)(\d)(\d)', 'abc')     ('', '', '')     '''     import re     m = re.search(regexp, s)     if m:         return m.groups()     return ('',) * num_of_groups(regexp) 

This allows me to do stuff like:

first, last, phone = groups(r'(\w+) (\w+) ([\d\-]+)', 'John Doe 555-3456') 

However, I don’t know how to implement num_of_groups. (Currently I just work around it.)

EDIT: Following the advice from rslite, I replaced re.findall with re.search.

sre_parse seems like the most robust and comprehensive solution, but requires tree traversal and appears to be a bit heavy.

MizardX’s regular expression seems to cover all bases, so I’m going to go with that.

Answer 1

def num_groups(regex):     return re.compile(regex).groups