Lookbehind Assertion is causing a "Invalid group" error

Question

I'm doing a simple Lookbehind Assertion to get a segment of the URL (example below) but instead of getting the match I get the following error:

Uncaught SyntaxError: Invalid regular expression: /(?<=\#\!\/)([^\/]+)/: Invalid group

Here is the script I'm running:

var url = window.location.toString();

url == http://my.domain.com/index.php/#!/write-stuff/something-else

// lookbehind to only match the segment after the hash-bang.

var regex = /(?<=\#\!\/)([^\/]+)/i; 
console.log('test this url: ', url, 'we found this match: ', url.match( regex ) );

the result should be write-stuff.

Can anyone shed some light on why this regex group is causing this error? Looks like a valid RegEx to me.

I know of alternatives on how to get the segment I need, so this is really just about helping me understand what's going on here rather than getting an alternative solution.

Thanks for reading.

J.

Can you give some example input string and what part of it must be matched by regex? — Shekhar
– Shekhar, Commented May 12, 2011 at 5:36
I've moved it from within the code block to the main example. Updated above. Please let me know if you'd like more detail and I will happily post it. — Jannis
– Jannis, Commented May 12, 2011 at 5:45

Trott · Accepted Answer · 2011-05-12 05:50:23Z

12

I believe JavaScript does not support positive lookbehind. You will have to do something more like this:

<script>
var regex = /\#\!\/([^\/]+)/;
var url = "http://my.domain.com/index.php/#!/write-stuff/something-else";
var match = regex.exec(url);
alert(match[1]);
</script>

answered May 12, 2011 at 5:50

Trott

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mVChr · Accepted Answer · 2011-05-12 05:51:33Z

7

Javascript doesn't support look-behind syntax, so the (?<=) is what's causing the invalidity error. However, you can mimick it with various techniques: http://blog.stevenlevithan.com/archives/mimic-lookbehind-javascript

answered May 12, 2011 at 5:51

mVChr

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SridharKritha · Accepted Answer · 2020-07-15 07:12:12Z

0

Also you could use String.prototype.match() instead of RegExp.prototype.exec() in the case of global(/g) or sticky flags(/s) are not set.

var regex = /\#\!\/([^\/]+)/;
var url = "http://my.domain.com/index.php/#!/write-stuff/something-else";
var match = url.match(regex); // ["#!/write-stuff", "write-stuff", index: 31, etc.,]
console.log(match[1]); // "write-stuff"

answered Jul 15, 2020 at 7:12

SridharKritha

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DarkBee · Accepted Answer · 2025-02-11 08:21:57Z

0

Lookbehind assertion has been supported in all the latest browsers for a while now

Documentation

edited Feb 11 at 8:21

DarkBee

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answered Feb 5 at 8:56

Max Leizerovich

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2 Comments

Max Leizerovich Feb 11 at 8:17

@DarkBee - I don't see any answer here pointing out that this is now supported in multiple browsers.

DarkBee Feb 11 at 8:21

Seems I've read your answer to diagonal. It would help though to add some code to demonstrate this effect

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