ContainsWordGrammar

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ContainsWord
{
    class Grammar
    {
        public string[] Variables { get; private set; }
        public char[] Terminals { get; private set; }
        public Rule[] Rules { get; private set; }
        public string StartSymbol { get; private set; }

        public Grammar(string variables, string terminals, string rules, string start)
        {
            Variables = variables.Substring(1, variables.Length - 2).Split(',');
            Terminals = Array.ConvertAll(terminals.Substring(1, terminals.Length - 2).Split(','), s => s[0]);
            Rules = Rule.Create(rules.Substring(1, rules.Length - 2).Split(','));
            StartSymbol = start;
        }
    }

    class Rule
    {
        public static Rule[] Create(string[] rules)
        {
            var arr = new Rule[rules.Length];
            for (var i = 0; i < rules.Length; i++)
            {
                arr[i] = new Rule(rules[i]);
            }
            return arr;
        }

        public string Input { get; private set; }
        public string Output { get; private set; }

        private Rule(string rule)
        {
            var parts = rule.Split(new string[] { "->" }, StringSplitOptions.None);
            Input = parts[0];
            Output = parts[1];
        }

        public void Use(string form, List<string> set, string target)
        {
            var matches = System.Text.RegularExpressions.Regex.Matches(form, Input);
            foreach (System.Text.RegularExpressions.Match match in matches)
            {
                var result = match.Index + 1 < form.Length ? form.Substring(0, match.Index) + Output + form.Substring(match.Index + Input.Length) :
                    form.Substring(0, match.Index) + Output;
                if (result.Length <= target.Length && !set.Contains(result))
                    set.Add(result);
            }
        }
    }
}