如何在Angular 7中验证带+号的国家代码？

import { AbstractControl, ValidatorFn } from '@angular/forms';

export function countryCodeValidator(): ValidatorFn {
  return (control: AbstractControl): { [key: string]: any } | null => {
    const countryCode = control.value;
    const isValid = /^\+\d+$/.test(countryCode);
    return isValid ? null : { invalidCountryCode: { value: countryCode } };
  };
}

<form [formGroup]="myForm">
  <input type="text" formControlName="countryCode">
  <div *ngIf="myForm.get('countryCode').errors?.invalidCountryCode">
    Invalid country code. Please enter a valid country code starting with '+'.
  </div>
</form>

import { Component } from '@angular/core';
import { FormBuilder, FormGroup, Validators } from '@angular/forms';
import { countryCodeValidator } from './country-code.validator';

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.css']
})
export class MyComponent {
  myForm: FormGroup;

  constructor(private fb: FormBuilder) {
    this.myForm = this.fb.group({
      countryCode: ['', [Validators.required, countryCodeValidator()]]
    });
  }
}