How do I match a number inside square brackets with regex

Question

I wrote a regular expression which I expect should work but it doesn't.

  var regex = new RegExp('(?<=\[)[0-9]+(?=\])')

Javascript is giving me the error Invalid regular expression :(/(?<=[)[0-9]+(?=])/): Invalid group

Does javascript not support lookahead or lookbehind?

Answer 1

Lookahead is supported, but not lookbehind. You can get close, with a bit of trickery.

Answer 2

If you're quoting a RegExp, watch out for double escaping your backslashes.

Answer 3

This should work:

var regex = /\[[0-9]+\]/;

---

edit: with a grouping operator to target just the number:

var regex = /\[([0-9]+)\]/;

With this expression, you could do something like this:

var matches = someStringVar.match(regex);
if (null != matches) {
  var num = matches[1];
}