validation - PHP generating invalid strings

Question

There's a lot of implementations for generating random strings that start with a string of allowable characters to pick from. I want to do the reverse and provide an alphabet of characters to exclude (all others being allowed).

The purpose of this is to test rejecting invalid form input so it does not need to be cryptographically secure, just sufficiently random. I could enumerate bunch of invalid entries but risk missing something. Not looking to test everything in one go - each test run could generate a handful of invalid strings to assert, and over time cover more of the full gamut.

Something like below where $valid contains allowable characters to exclude. Not sure if there's a faster/easier way to do this.

function randomInvalidChar($valid = '') {
    do {
        $ord = mt_rand(0, 1112063);            // size of utf-8 character set
        $char = mb_chr($ord, 'UTF-8');
        $isValid = mb_strpos($valid, $char);
    } while ( $isValid !== false );

    return $char;
}

$random_string = randomInvalidChar('1234567890.+-*/^');

// => "?"

question from:https://stackoverflow.com/questions/65600424/php-generating-invalid-strings

Answer 1

Waitting for answers