Why does the regular expression /[_-%]/ break JavaScript?

The following JavaScript outputs nothing (not even "false"), and indeed stops any other JavaScript on the page from 开发者_开发问答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.

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.

It's because in the interpreter thinks it's dealing with a range. Just like /[a-z]/ will match any character between a and z, /[_-%]/ will (try to) match any character between _ and %. This doesn't make sense, so JavaScript stops. Putting the hyphen as the first or last character will fix the issue.

It's the dash in the middle. JavaScript treats [_-%] as a character class range, similar to [A-Z]. I'm guessing the browser you're using simply doesn't handle this case very well. I suggest moving the hyphen to the front, or escaping it with a backslash (\-).

Use the following instead:

/[-_%]/

The - would have been interpreted to be a range. You can also use \-