使用由发布的正则表达式Moaz （并增加资本），我们需要加入registration_errors 过滤器：

// Restrict username registration to alphanumerics
add_filter(\'registration_errors\', \'limit_username_alphanumerics\', 10, 3);
function limit_username_alphanumerics ($errors, $name) {

 if ( ! preg_match(\'/^[A-Za-z0-9]{3,16}$/\', $name) ){
    $errors->add( \'user_name\', __(\'<strong>ERROR</strong>: Username can only contain alphanumerics (A-Z 0-9)\',\'CCooper\') );
 }
 return $errors;
}