Question

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Interesting test of Javascript RegExp
Regular expression test can't decide between true and false (JavaScript)

Example of issue. When ran inline the results are as I would expect. But when stored as a variable it skips the middle span element.

// Inline RegExp
function getToggleClasses() {
  var toggler = [],
      elements = document.getElementsByTagName("*"),
      i=0,
      len = elements.length;

  for (i; i < len; i++) {
    if (/toggler/g.test(elements[i].className)) {
      toggler.push(elements[i]);
    }
  }

  document.getElementById('results').innerHTML += "<br />Inline: " + toggler.length;
}

// Variable
function getToggleClasses2() {
  var toggler = [],
      elements = document.getElementsByTagName("*"),
      tester = /toggler/g,
      i=0,
      len = elements.length;

  for (i; i < len; i++) {
    if (tester.test(elements[i].className)) {
      toggler.push(elements[i]);
    }
  }

  document.getElementById('results').innerHTML += "<br />Variable: " + toggler.length;
}
​

Mark up:

<span class="toggler">A</span>
<span class="toggler">B</span>
<span class="toggler">C</span>

Given:
I understand there is no reason to use a RegExp to do this comparison and I also understand how great libraries such as jQuery are. I also know that the g is not needed in this case.

I can’t understand why these two methods should ever return different results.

Answer 1

RegExp instances are stateful, so reusing them can cause unexpected behavior. In this particular case, it’s because the instance is global, meaning:

that the regular expression should be tested against all possible matches in a string.

That’s not the only difference caused by using g, however. From RegExp.test @ MDN:

As with exec (or in combination with it), test called multiple times on the same global regular expression instance will advance past the previous match.

---

Remove the g flag, or set lastIndex to 0 (thanks, @zzzzBov).