phpregexlookbehindvariable-width

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

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+/mi

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

    Use \V+ to match the remaining non-vertical (such as \r and \n) characters excluding in the line.

    See the broader canonical: Variable-length lookbehind-assertion alternatives for regular expressions

    To protect your script from falsely matching values instead of matching labels, notice the use ^ with the m modifier. This will ensure that you are matching labels that occur at the start of a line.

    Without a start of line anchor, Somethingelse: url whoops will match whoops.

    To make multiple matches in PHP, the g pattern modifier is not used. Instead, apply the pattern in preg_match_all()