Adventure of code 2020 day 19 solution in Python and regex

aoc202019.py

import regex
def defs(rules_s):
    for rule in rules_s.split('\n'):
        n,s = rule.split(': ',1)
        s = regex.sub(r'\s*(\d+)\s*',r'(?&r\1)',s)
        s = regex.sub(r'"(\w+)"',r'\1',s)
        yield "(?P<r{}>{})".format(n,s)
rules_s, messages_s = open('./input').read().split("\n\n",1)
r=regex.compile(r'(?(DEFINE){})^(?&r0)$'.format(''.join(defs(rules_s))))
print("part1:", sum(1 if r.match(msg) else 0 for msg in messages_s.split('\n')))
rules_s, messages_s = open('./input').read().split("\n\n",1)
rules_s = regex.sub(r'8: 42','8: 42 | 42 8',rules_s)
rules_s = regex.sub(r'11: 42 31','11: 42 31 | 42 11 31',rules_s)
# .. or replace with what you had in the instructions page
r=regex.compile(r'(?(DEFINE){})^(?&r0)$'.format(''.join(defs(rules_s))))
print("part2:", sum(1 if r.match(msg) else 0 for msg in messages_s.split('\n')))