How do you implement custom form validation in

ngular?

Create a validator function returning an error object or null.

✅ Example (Custom validator checking forbidden name):

function forbiddenNameValidator(nameRe: RegExp): ValidatorFn {

return (control: AbstractControl): ValidationErrors | null => {
const forbidden = nameRe.test(control.value);
return forbidden ? { forbiddenName: { value: control.value } } :

null;

};

}

// Usage:

new FormControl('', [forbiddenNameValidator(/admin/i)])