day19_part2.nim

import strutils, tables, nre, strformat

proc parseRules(rules: seq[string]): Table[string, string] =
  for rule in rules:
    var parts = rule.split(":")
    result[parts[0]] = parts[1].strip

proc buildRegex(idx: string, rules: Table[string, string]): string =
  let rule = case idx:
    of "8": "(?:42)+"
    of "11": "(?<x>42 31 | 42 \\g<x> 31)"
    else: rules[idx]

  if (let rm = rule.match(re("\"([a-z])\"")); rm.isSome):
    rm.get.captures[0]
  else:
    replace(&"(?:{rule})", re"\d+", proc(match: string): string =
      buildRegex(match, rules))

let
  input    = readAll(stdin).split("\n\n")
  rules    = parseRules(input[0].strip.splitLines)
  messages = input[1].strip.splitLines
  regex    = re("(?x)" & buildRegex("0", rules) & "\\z")

var count = 0
for message in messages:
  if (let rm = message.match(regex); rm.isSome):
    count.inc

echo count