JavaScript

var regexObj = /^\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$/;
if (regexObj.test(subjectString)) {
    var formattedPhoneNumber =
        subjectString.replace(regexObj, "($1) $2-$3");
} else {
    // Ungültige Telefonnummer
}