Regex Error: A lookbehind assertion has to be fixed width

My pattern works in JavaScript.

(?<=(?:username|Email|URL)(?:\s|:))[^\n]+

However, when I try to use it in PHP, I get this error:

A lookbehind assertion has to be fixed width

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How I can fix it?

Demo: https://regex101.com/r/x2W3S5/1

>Solution :

Use a full string match restart (\K) instead of the invalid variable-length lookbehind.

Regex 101 Demo

(?:username|Email|Url):? *\K[^\v]+

Make the colon and space optional by trailing them with ? or *.

See also: Variable-length lookbehind-assertion alternatives for regular expressions