how to create custom validator function
Step 1 : Create the custom validator function function emailDomain(control: AbstractControl): { [key: string]: any } | null { const email: string = control.value; const domain = email.substring(email.lastIndexOf( '@' ) + 1 ); if (email === '' || domain.toLowerCase() === 'gmail.com' ) { return null ; } else { return { 'emailDomain' : true }; } } Step 2 : Attach the custom validator function to the control that we want to validate email: [ '' , [ Validators .required, emailDomain ]] Step 3 : Display the validation error message <span * ngIf = "employeeForm.get('email').errors.emailDomain" > Email domian should be gmail.com </span>