c# - DataValidation using Regex not working -

so have maskedtextinput box follows:

<telerik:radmaskedtextinput margin="2" borderbrush="lightgray"                             isclearbuttonvisible="false"                             mask="(###) ###-####"                             updatevalueevent="propertychanged"                             textmode="plaintext"                             value="{binding path=phonenumber,                                              mode=twoway,                                              notifyonvalidationerror=true,                                              validatesonexceptions=true,                                             updatesourcetrigger=propertychanged}" /> 

and bound through viewmodel following code-behind:

//phone number [required(allowemptystrings = false, errormessage = @"a phone number required.")] [regularexpression(@"^\d{10}$", errormessage = @"invalid phone number.")] public string phonenumber {     { return _phonenumber; }     set     {         if (_phonenumber!= value)         {             validator.validateproperty(value,                  new validationcontext(this, null, null)                 {                     membername = "phonenumber"                 });             _phonenumber = value;             onpropertychanged("phonenumber");         }     } } 

what i'm attempting when user has other 10 digits present throw exception causing border turn red.

it works fine other fields email address reason not work phone number field mask set.

as per documentation telerik need throw exception in set statement.

set {     if (_phonenumber!= value)     {         // validation , throw validation exception.         if(/*validation statements*/)         {           throw new validationexception("........error message..........");         }         _phonenumber = value;         onpropertychanged("phonenumber");     } }