PHP通过正则表达式对用户名合法性校验

$users = [
    '梅子16352_aa',
    'www',
    'eeeee.',
    'ee-',
    'goodman',
];

// 支持4-20位大小写字母和下划线
function isUsername($string)
{
    return preg_match('/^[A-Za-z0-9_]{4,20}$/', $string);
}

foreach ($users as $user) {
    var_dump(isUsername($user));
}

/**
int(0)
int(0)
int(0)
int(0)
int(1)
 */