Exclude files using a regular expression

Question

i want to exclude all .mp3 and .jpeg files (with a regular expression, not PHP). I know about !preg_match )

My example below always matches:

$str = 'file.mp3'; // Exclude
$ex  = '~(?!\.(mp3|jpe?g))$~';

if (preg_match($ex, $str)) {
    echo "Match!";
} else {
    echo "Nothing Match!";
}

Answer 1

Your negative lookahead isn't working because there is nothing to look ahead at. Remember that lookaround assertions are zero-width — they do not actually consume characters. You will still need to account for the filename extension characters.

Change the expression as follows:

$ex = '~(?!\.(mp3|jpe?g))[a-z]{3,4}$~';

Demo

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A better approach would be to use pathinfo() though. Maintain an array of extensions that you'd like to disallow and then use in_array() to check if the extension of the filename is in that array:

$disallowed = ['mp3', 'jpg', 'jpeg', /* more extensions */ ];

if (in_array(pathinfo($str, PATHINFO_EXTENSION), $disallowed)) {
    # code...
}