Firefox error: Unable to check input because the pattern is not a valid regexp: invalid identity escape in regular expression

Question

I'm using regex pattern matching for HTML5 form validation. The latest version of Firefox gives me an error. I only started seeing this in Firefox 46. I don't think this was a problem in earlier Firefox versions.

Unable to check <input pattern='[\@\%]'> because the pattern is not a valid regexp: invalid identity escape in regular expression

Caused by this very simple test case:

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
  </head>
  <form>
    <input pattern="[\@\%]">
  </form>
</html>

Why is escaping these characters considered an error? I've always escaped everything in my regular expressions that isn't a number or a letter. I've never had anything complain this type of escaped character except this version of Firefox.

When I learned regex, I was told that everything that wasn't a number or a letter could have special meaning. Even if it doesn't now, it might in a future version, so it is better to escape them. Is this not true?

Is there a list of characters I shouldn't escape for Firefox?

sidenote :- you don't need to escape those characters in character class as they don't have any special meaning — rock321987
– rock321987, Commented Apr 30, 2016 at 10:38

Mathias Bynens · Accepted Answer · 2016-05-01 09:11:45Z

13

This is due to the following change: Bug 1227906 - HTML pattern attribute should set u flag for regular expressions

As someone has already said, you don't have to escape those characters. Just use:

<input pattern="[@%]">

edited May 1, 2016 at 9:11

Mathias Bynens

151k54 gold badges224 silver badges254 bronze badges

answered Apr 30, 2016 at 14:51

Kohei Yoshino

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6 Comments

Kohei Yoshino Over a year ago

Posted the site compatibility doc for this.

sideshowbarker Over a year ago

The related github issue has also now been reopened.

Stephen Ostermiller Over a year ago

Any idea why you can't escape those characters in Unicode regex mode?

Mathias Bynens Over a year ago

@StephenOstermiller It’s not necessary to escape them, so why would you? The reason unnecessary escapes throw in u mode is so that the spec can later assign a special meaning to some of them, such as \p and \P. That would be a backwards-incompatible change if \p were equivalent to just p.

Stephen Ostermiller Over a year ago

Escaped numbers and letters are reserved for special use, but my understanding is that all symbols are reserved for future use unescaped.

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