title: ngx-mask to accept negative numbers
body: <p>We are using MaskDirective &amp; MaskService from ngx-mask in order to make users accept input currency in the following format: 000000.00.</p>
  
  <p>The following is my directive. I'm using input type="decimal".
  Now I have a scenario wherein one situation this directive should accept a negative number (if there is one).</p>
  
  <p>How can I implement this? Tried using "specialCharacters" but I couldn't get through.</p>
  
  <p>Any help is highly appreciated. Thanks in advance.</p>
  
  <pre><code>`@Directive({
    selector: 'input[type=decimal]',
    providers: [
      {
        provide: NG_VALUE_ACCESSOR,
        useExisting: forwardRef(() =&gt; DecimalDirective),
        multi: true,
      },
      MaskService,
    ],
  })
  export class DecimalDirective extends MaskDirective implements OnInit {
    @HostBinding('type') public type = 'text';
  
    ngOnInit() {
      this.maskExpression = '099999.00';
      this.dropSpecialCharacters = false;
    }
  }`
  </code></pre>
  
  <p>Usage:</p>
  
  <pre><code>`&lt;input type="decimal" name="sampleAmount" formControlName="sampleAmount" class="form-control" placeholder="0.00"&gt;`
  </code></pre>