System.Text.RegularExpressions.Match.GroupToStringImpl C# (CSharp) Method

        internal virtual string GroupToStringImpl(int groupnum)
        {
            int c = _matchcount[groupnum];
            if (c == 0)
                return string.Empty;

            int[] matches = _matches[groupnum];

            return _text.Substring(matches[(c - 1) * 2], matches[(c * 2) - 1]);
        }

 /// <summary>
 /// Given a Match, emits into the StringBuilder the evaluated
 /// substitution pattern.
 /// </summary>
 private void ReplacementImpl(StringBuilder sb, Match match)
 {
     for (int i = 0; i < _rules.Count; i++)
     {
         int r = _rules[i];
         if (r >= 0)   // string lookup
             sb.Append(_strings[r]);
         else if (r < -Specials) // group lookup
             sb.Append(match.GroupToStringImpl(-Specials - 1 - r));
         else
         {
             switch (-Specials - 1 - r)
             { // special insertion patterns
                 case LeftPortion:
                     sb.Append(match.GetLeftSubstring());
                     break;
                 case RightPortion:
                     sb.Append(match.GetRightSubstring());
                     break;
                 case LastGroup:
                     sb.Append(match.LastGroupToStringImpl());
                     break;
                 case WholeString:
                     sb.Append(match.GetOriginalString());
                     break;
             }
         }
     }
 }