Find a percentage value in a string using preg_match

if (preg_match("/[0-9]+%/", $string, $matches)) {
    $percentage = $matches[0];
    echo $percentage;
}

use the regex /([0-9]{1,2}|100)%/. The {1,2} specifies to match one or two digits. The | says to match the pattern or the number 100.

[0-99] which you had matches one character in the range 0-9 or the single digit 9 which is already in your range.

Note: This allows 00, 01, 02, 03...09 to be valid. If you do not want this, use /([1-9]?[0-9]|100)%/ which forces one digit and an optional second in the range 1-9

Why not /\d+%/? Short and sweet.