Regex conversion from PHP to JavaScript

Question

Could you please help me to convert to JavaScript below PHP regex that is working in following filter?

function custom_filter_wpcf7_is_tel( $result, $tel ) { 
$result = preg_match( '/^((\(?\+45\)?)?)(\s?\d{2}\s?\d{2}\s?\d{2}\s?\d{2})$/', $tel );
return $result; 
}

add_filter( 'wpcf7_is_tel', 'custom_filter_wpcf7_is_tel', 10, 2 );

When I put this into desired jQuery code:

$.validator.addMethod(
    "regex",
    function(value, element, regexp) {
        var re = new RegExp(regexp);
        return this.optional(element) || re.test(value);
    },
    "Indtast et gyldigt telefonnummer."
);

$( "#lt_phone" ).rules( "add", {
    regex: "/^((\(?\+45\)?)?)(\s?\d{2}\s?\d{2}\s?\d{2}\s?\d{2})$/",
});

I've got invalid regexp group error.

I tried to edit my regex in a different ways using regex101.com and some googling but with no luck.