Confirm password validation in Angular 6

You can simply use password field value as a pattern for confirm password field. For Example :

<div class="form-group">
 <input type="password" [(ngModel)]="userdata.password" name="password" placeholder="Password" class="form-control" required #password="ngModel" pattern="(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{8,}" />
 <div *ngIf="password.invalid && (myform.submitted || password.touched)" class="alert alert-danger">
   <div *ngIf="password.errors.required"> Password is required. </div>
   <div *ngIf="password.errors.pattern"> Must contain at least one number and one uppercase and lowercase letter, and at least 8 or more characters.</div>
 </div>
</div>

<div class="form-group">
 <input type="password" [(ngModel)]="userdata.confirmpassword" name="confirmpassword" placeholder="Confirm Password" class="form-control" required #confirmpassword="ngModel" pattern="{{ password.value }}" />
 <div *ngIf=" confirmpassword.invalid && (myform.submitted || confirmpassword.touched)" class="alert alert-danger">
   <div *ngIf="confirmpassword.errors.required"> Confirm password is required. </div>
   <div *ngIf="confirmpassword.errors.pattern"> Password & Confirm Password does not match.</div>
 </div>
</div>