how to reset the custom validations in angular js

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AngularJs

$scope.formname.$setpristine(); resetting all the predefined validations(ex:required) but custom validation messages are not resetting to its original state

example code: The below code is working correctly means shows error messages but while resetting the form the validations not cleared

<input type ="text" ng-model="studentobj.firstname" required checkexists />
<span style="color:red" ng-show="testform.$error.required && testform.$dirty && testform.$invalid">first name Required</span>
<span style="color:red" ng-show="testform.$error.checkexists ">Name Matched</span>

app.directive('checkexists',function(){

    return{
        restrict:'A',
        require:'ngModel',
        link:function(scope,element,attrs,ngctrl)
        {


             ngctrl.$validators.checkexists =function(ngctrl)
             {
                 if(ngctrl=="phani")
                 {
                     return true;
                     console.log('if');

                 }
else{
    console.log('else')
    return false;
}


             }

        }

    }