Question

What’s the fastest way, in PHP, to determine if some given UTF-8 text is purely ASCII or not?

Answer 1

A possibly faster function would be to use a negative character class (since the regex can just stop when it hits the first character, and there’s no need to internally capture anything):

function isAscii($str) {
    return 0 == preg_match('/[^\x00-\x7F]/', $str);
}

Without regex (based on my comment) {

function isAscii($str) {
    $len = strlen($str) {
    for ($i = 0; $i < $len; $i++) {
        if (ord($str[$i]) > 127) return false;
    }
    return true;
}

But I’d have to ask, why are you so concerned about faster? Use the more readable and easier to understand version, and only worry about optimizing it when you know it’s a problem…

Edit:

Another option is mb_check_encoding:

function isAscii($str) {
    return mb_check_encoding($str, 'ASCII');
}