System.Text.RegularExpressions.RegexBoyerMoore.IsMatch C# (CSharp) Method

        internal bool IsMatch(string text, int index, int beglimit, int endlimit)
        {
            if (!_rightToLeft)
            {
                if (index < beglimit || endlimit - index < _pattern.Length)
                    return false;

                return MatchPattern(text, index);
            }
            else
            {
                if (index > endlimit || index - beglimit < _pattern.Length)
                    return false;

                return MatchPattern(text, index - _pattern.Length);
            }
        }

        protected override bool FindFirstChar()
        {
            int    i;
            String set;

            if (0 != (runanchors & (RegexFCD.Beginning | RegexFCD.Start | RegexFCD.EndZ | RegexFCD.End)))
            {
                if (!runcode._rightToLeft)
                {
                    if ((0 != (runanchors & RegexFCD.Beginning) && runtextpos > runtextbeg) ||
                        (0 != (runanchors & RegexFCD.Start) && runtextpos > runtextstart))
                    {
                        runtextpos = runtextend;
                        return(false);
                    }
                    if (0 != (runanchors & RegexFCD.EndZ) && runtextpos < runtextend - 1)
                    {
                        runtextpos = runtextend - 1;
                    }
                    else if (0 != (runanchors & RegexFCD.End) && runtextpos < runtextend)
                    {
                        runtextpos = runtextend;
                    }
                }
                else
                {
                    if ((0 != (runanchors & RegexFCD.End) && runtextpos < runtextend) ||
                        (0 != (runanchors & RegexFCD.EndZ) && (runtextpos < runtextend - 1 ||
                                                               (runtextpos == runtextend - 1 && CharAt(runtextpos) != '\n'))) ||
                        (0 != (runanchors & RegexFCD.Start) && runtextpos < runtextstart))
                    {
                        runtextpos = runtextbeg;
                        return(false);
                    }
                    if (0 != (runanchors & RegexFCD.Beginning) && runtextpos > runtextbeg)
                    {
                        runtextpos = runtextbeg;
                    }
                }

                if (runbmPrefix != null)
                {
                    return(runbmPrefix.IsMatch(runtext, runtextpos, runtextbeg, runtextend));
                }
            }
            else if (runbmPrefix != null)
            {
                runtextpos = runbmPrefix.Scan(runtext, runtextpos, runtextbeg, runtextend);

                if (runtextpos == -1)
                {
                    runtextpos = (runcode._rightToLeft ? runtextbeg : runtextend);
                    return(false);
                }

                return(true);
            }

            if (runfcPrefix == null)
            {
                return(true);
            }

            runrtl = runcode._rightToLeft;
            runci  = runfcPrefix.CaseInsensitive;
            set    = runfcPrefix.Prefix;

            if (RegexCharClass.IsSingleton(set))
            {
                char ch = RegexCharClass.SingletonChar(set);

                for (i = Forwardchars(); i > 0; i--)
                {
                    if (ch == Forwardcharnext())
                    {
                        Backwardnext();
                        return(true);
                    }
                }
            }
            else
            {
                for (i = Forwardchars(); i > 0; i--)
                {
                    if (RegexCharClass.CharInSet(Forwardcharnext(), set, String.Empty))
                    {
                        Backwardnext();
                        return(true);
                    }
                }
            }
            return(false);
        }