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Editorial Team
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Asked: 2026-05-22T22:01:29+00:00

Is it possible to do a preg_match() on something that shouldn’t be a match

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Is it possible to do a preg_match() on something that shouldn’t be a match whilst still returning true?

For example, at the moment we have…

if (preg_match('#^Mozilla(.*)#', $agent)) {

We want to check if the Mozilla string is not in $agent, but still have preg_match return true.

We can’t change it to:

if (!preg_match('#^Mozilla(.*)#', $agent)) {
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1 Answer

  1. Editorial Team

    What you want is a negative lookahead, and the syntax is:

    if (preg_match('#^(?!Mozilla).#', $agent)) {

    Actually, you can probably get away with just #^(?!Mozilla)# for this. I don’t know how PHP will feel about a pattern that’s nothing but zero-width tokens, but I’ve tested it in JavaScript and it works fine.

    Edit:

    If you want to make sure Mozilla doesn’t appear anywhere in the string, you could use this…

    if (preg_match('#^((?!Mozilla).)*$#', $agent)) {

    …but only if you can’t use this!

    if (strpos($agent, 'Mozilla') !== false) {
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