No need for regex

For simple operations like validating a username, there is no need to use regex. Instead, use PHP’s built in functions from the Ctype-library.

$testcase = 'AbCd1zyZ9')

if (ctype_alnum($testcase)) {
    echo "The string $testcase consists of all letters or digits.";
}

…check for digits with ctype_digit


$testcase = "12345";

if (ctype_digit($testcase)) {
    echo "The string $testcase consists of all digits.";
}

…or look for visible characters with ctype_graph


$testcase = "asdf\n\r\t";

if (ctype_graph($testcase)) {
    echo "The string consists of all (visibly) printable characters.";
} else {
    echo "Contains invisible characters.";
}

// RESULT: Contains invisible characters.