Simulate a right-to-left regex with negative lookahead and backref

reverso.py

import re

def get_matches(regex, mystring):
    return [i.group(0) for i in re.finditer(regex, mystring)]

input1 = 'A Foo, A Foo qux: Foo qux: Foo qux:'
regex1 = r'Foo.*?qux:'

print get_matches(regex1, input1)
# >>> ['Foo, Foo qux:', 'Foo qux:', 'Foo qux:']
# Oops, not quite.

input2 = input1[::-1] # reverse the string
regex2 = r':xuq.*?ooF'

print [i[::-1] for i in get_matches(regex2, input2)]
# >>> ['Foo qux:', 'Foo qux:', 'Foo qux:']

# OK... but that's a dumb, ugly hack.

# Better: use negative lookahead with the backreference
regex2 = r'(Foo)(?:(?!\1).)*qux:'
# (?: ) is a non-capturing group... a capturing group would
#       work just as well but is less efficient. 
# (?! ) is a negative lookahead
# \1    is the backreference to the first captured group
#
# Net result: Capture all Foo.*qux: except those that have
#             a second 'Foo' in between.

print get_matches(regex2, input1)
# >>> ['Foo qux:', 'Foo qux:', 'Foo qux:']