code: function locator(value, fromIndex) {
      var result
      var prev
  
      regex.lastIndex = fromIndex
  
      result = regex.exec(value)
  
      if (result) {
        result = regex.lastIndex - result[0].length
        prev = value.charCodeAt(result - 1)
  
        if (usernameCharacter(prev) || prev === CC_HASH || prev === CC_AT) {
          /* Find the next possible value. */
          return locator(value, regex.lastIndex)
        }
  
        return result
      }
  
      return -1
    }
docs: /* Find the place where a regex begins.