adding_custom_validation.ts

const userForm = new FormGroup({
 userName: new FormControl('', [ 
     Validators.required, 
     Validators.minLength(2),
     forbiddenNameValidator(/bob/i) // <-- Here's how you pass in the custom validator.
 ]),
 firstName: new FormControl('', [ Validators.required, Validators.minLength(2)]),
 lastName: new FormControl(''),
 address: new FormGroup({
   street: new FormControl('', Validators.required),
   city: new FormControl(''),
   zipCode: new FormControl('')
 })
});