var result = (function(x) {

  var matches,

      regex = /[\w]/gi;

  // when the regular expression is set on a variable or object a `lastIndex` property is set on it behind the scenes. So when the regular express contains the g (global) flag, it basically returns an array of matches and lastIndex will contain the index of the last match.

  console.log(regex.lastIndex); // 0

  matches = regex.test(x);

  console.log(matches); // true

  console.log(regex.lastIndex); // 1

  matches = regex.test(x);

  console.log(matches); // false, because there are no more matches since the value we tested with is only one letter and it already has been matched.

  return matches;

})('a');

​

console.log(result); // false