Angular 6 Directive to allow only positive integers in an input

number-only.directive.ts

// *** NOTE: This is only allowing positive integers, adjust the regex to suit your needs
// Stackblitz (w/ Angular Material): https://stackblitz.com/edit/angular-8fsecv?file=app%2Fnumber-only.directive.ts

import { Directive, ElementRef, HostListener } from '@angular/core';

@Directive({
  selector: '[numberOnly]'
})
export class NumberOnlyDirective {
  // Only want positive integers
  private regex: RegExp = new RegExp(/^\d+$/g);
  // Allow key codes for special events Backspace, tab, end, home
  private specialKeys = ['Backspace', 'Tab', 'End', 'Home'];

  constructor(private el: ElementRef) { }
  @HostListener('keydown', ['$event'])
  onKeyDown(event: KeyboardEvent) {
    if (this.specialKeys.indexOf(event.key) >= -1) {
      return;
    }
    let current: string = this.el.nativeElement.value;
    let next: string = current.concat(event.key);
    if (next && !String(next).match(this.regex)) {
      event.preventDefault();
    }
  }
}