Angular - Custom directive to accept only positive numbers within inputs

demo.html

<!-- keep it as text to hide arrow up and down -->
<!-- user can add negative numbers if type number -->

<input type="text" positiveNumbers>

positive-numbers.directive.ts

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

@Directive({
  selector: '[positiveNumbers]'
})
export class PositiveNumbersDirective {

  private regex: RegExp = new RegExp(/^-?[0-9]+(\.[0-9]*){0,1}$/g);
  private specialKeys: Array<string> = ['Backspace', 'Tab'];

  constructor(private el: ElementRef) {
  }

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

positive-numbers.module.ts

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { PositiveNumbersDirective } from './positive-numbers.directive';

@NgModule({
  declarations: [
    PositiveNumbersDirective
  ],
  imports: [
    CommonModule
  ],
  exports: [
    PositiveNumbersDirective
  ]
})
export class PositiveNumbersModule { }