title: MongoDB query to find out documents with phone numbers which are not in XXX-XXX-XXXX format
body: <p>I have a collection in which documents have "phone" string field in XXX-XXX-XXXX this format. but there are some phone numbers in different format so we want to find out that documents which have different phone format. 
  MongoDB query to find out documents with phone numbers which are not in XXX-XXX-XXXX format.</p>