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Asked: 2026-05-13T20:01:59+00:00

What does (?<=x) mean in regex? By the way, I have read the manual

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What does (?<=x) mean in regex?

By the way, I have read the manual here.

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  1. Editorial Team

    It’s a positive lookbehind.

    (?<=a)b (positive lookbehind) matches the b (and only the b) in cab, but does not match bed or debt.

    Update 2022:

    As of 2020 most major browsers except Safari have added support for lookbehind expressions. You can see the full support table here https://caniuse.com/js-regexp-lookbehind. However do note that other languages may not as noted in the original answer below. The updated quote from the linked manual is:

    Finally, flavors like std::regex and Tcl do not support lookbehind at all, even though they do support lookahead.

    Original answer:

    You won’t find it in any JavaScript manual because it’s not supported in JavaScript regex:

    Finally, flavors like JavaScript, Ruby and Tcl do not support lookbehind at all, even though they do support lookahead.

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