code: function _indexOfGroup(match, group_number) {
          var index = 0,
              i;
  
          for (i = 1; i < group_number; ++i) {
              if (match[i]) {
                  index += match[i].length;
              }
          }
  
          return index;
      }
docs: finds out the position of group match for a regular expression
  
  @see http://stackoverflow.com/questions/1985594/how-to-find-index-of-groups-in-match
  
  @param {Object} match
  @param {number} group_number
  @returns {number}