code: function $(tok) {
          var tokType = typeof tok,
              match, length;
  
          // Either match a single character in the input,
          // or match a regexp in the current chunk (`current`).
          //
          if (tokType === "string") {
              if (input.charAt(i) !== tok) {
                  return null;
              }
              skipWhitespace(1);
              return tok;
          }
  
          // regexp
          sync ();
          if (! (match = tok.exec(current))) {
              return null;
          }
  
          length = match[0].length;
  
          // The match is confirmed, add the match length to `i`,
          // and consume any extra white-space characters (' ' || '\n')
          // which come after that. The reason for this is that LeSS's
          // grammar is mostly white-space insensitive.
          //
          skipWhitespace(length);
  
          if(typeof(match) === 'string') {
              return match;
          } else {
              return match.length === 1 ? match[0] : match;
          }
      }
docs: Parse from a token, regexp or string, and move forward if match