code: func (l *Lexer) indexRegexp(r *regexp.Regexp) int {
  	loc := r.FindStringIndex(l.input[l.pos:])
  	if loc == nil {
  		return -1
  	}
  	return loc[0]
  }
docs: // indexRegexp returns the index of the first string from current scanning position that matches given regular expression
  //
  // It returns -1 if not found