Regexp Basics - is it a letter?

[RazvanBugoi]

https://www.codewars.com/kata/567de72e8b3621b3c300000b/train/javascript

[RazvanBugoi]

String.prototype.isLetter = function() {
  return (/^[a-zA-Z]$/g).test(this);
}

String.prototype.isLetter = function() {
  return this.match(/^[a-zA-Z]$/g) != null && this.match(/^[a-zA-Z]$/g).length == 1 
}