Making a part of the mask optional

Till now, all the examples of input masks have shown that all the characters in a mask are mandatory. It is also possible that you can make a part of the mask optional with the use of the question mark (?) character. Anything listed after the question mark within a mask definition will be treated as an optional input. A common use case is displaying the phone number with an optional extension number, as follows:

<span>Phone Ext</span>
<p-inputMask mask="(999) 999-9999? x99999" [(ngModel)]="optional"     
  name="optionalmask" placeholder="(999) 999-9999? x99999">
</p-inputMask>

Once the user finishes the input by reaching the question mark character and blurs the component, the rest of the validation will be skipped. That is, the input up to that part won't be erased. For example, phone number inputs, such as (666) 234-5678 and (666) 234-5678? x1230 will be valid inputs for the mask's optional case.