Question

The following JavaScript outputs nothing (not even “false”), and indeed stops any other JavaScript on the page from running:

var pattern = new RegExp(/[_-%]/);
document.write(pattern.test("foo"));

What is it about this regular expression that does this? If any one of the three characters (_, -, or %) is removed, everything works normally. And if the order of the three characters is changed at all, everything works normally.

Answer 1

A hyphen in a [ ] block is used for ranges. So _ to % is invalid.
You can escape it:

var pattern = new RegExp(/[_\-%]/);

or move to the start:

var pattern = new RegExp(/[-_%]/);

or to the end:

var pattern = new RegExp(/[_%-]/);

Since regex knows that a hyphen at the start (or end, thanks BrunoLM!) means a literal hyphen and not a range.