JavaScript regex look behind alternative

Question

I am trying to match the following string into 2 groups using look behind but apparently, they are not supported in JavaScript.

Regex: ((?<=:).*(?=;))|((?<=,).*$) Online Demo

data:image/jpeg;base64,/abcd1234...

--> group1: image/jpeg
--> group2: /abcd1234...

Then I tried to use XRegExp library hoping it would support look behind but still no success.

var XRegExp = require("xregexp");
var base64 = "data:image/jpeg;base64,/abcd1234...";
base64 = XRegExp.matchRecursive(base64, '((?<=:).*(?=;))|((?<=,).*$)', 'g');

But I get the following error:

node_modules/xregexp/xregexp-all.js:3376
        new RegExp(generated.pattern, generated.flags),
        ^
SyntaxError: Invalid regular expression: /((?<=:).*(?=;))|((?<=,).*$)/: Invalid group at new RegExp (native)

Is there a way to run the regex using JavaScript's native regex parser probably by reversing the string?

Why are you trying to pick apart a data URL? Where is it coming from, and what do you plan to do with it? — user663031
– user663031, Commented Jul 7, 2017 at 5:28
@torazaburo My highest priority is to extract mime type from it — Node.JS
– Node.JS, Commented Jul 7, 2017 at 5:30
I would prefer to GET the URL with responseType set to Blob, then consult the type property of the resulting blob. — user663031
– user663031, Commented Jul 7, 2017 at 5:39
Why do you need a lookbehind? Can't you just do :(.*);|,(.*$)? — K. Kirsz
– K. Kirsz, Commented Jul 7, 2017 at 5:56
Look-behinds were added in the 2018 specification. They work in Chrome, but Firefox has a bug due to lack of implementation. — Lonnie Best
– Lonnie Best, Commented Dec 8, 2019 at 0:25

revo · Accepted Answer · 2017-07-07 06:01:39Z

1

Be more specific:

(?=[^:;]*;)([^:;]+)|([^;,]+)$

Live demo

Or use more JS:

var re = /:([^:;]+);|,(.*)$/g;
var str = "data:image/jpeg;base64,/abcd1234...";
var matches;
while ((matches = re.exec(str)) !== null) {
  console.log(matches[1] ? matches[1] : matches[2])
}

answered Jul 7, 2017 at 6:01

revo

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