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();
    }
  }
}

‘this.specialKeys.indexOf(event.key) >= -1 ’ should be changed to '‘this.specialKeys.indexOf(event.key) > -1 ’'，Otherwise, you can never enter any data.
In addition, if you want to match positive integers without zero. Your regular expression needs to be modified to 'new RegExp(/^[1-9]\d*$/g)'

to handle scientific notation (1e6), you might want to do
/^[+]?([1-9][0-9]*(?:[\.][0-9]*)?|0*\.0*[1-9][0-9]*)(?:[eE][+-][0-9]+)?$/