title: MongoDB use index in regular expression query
body: <p>I am using the official C# MongoDB driver.</p>
  
  <p>If I have an index on three elements {"firstname":1,"surname":1,"companyname":1} can I search the collection by using a regular expression that directly matches against the index value?</p>
  
  <p>So, if someone enters "sun bat" as a search term, I would create a regex as follows
  (?=.<em>\bsun)(?=.</em>\bbat).* and this should match any index entries where firstname or surname or companyname starts with 'sun' AND where firstname or surname or companyname starts with 'bat'.</p>
  
  <p>If I can't do it this way, how can I do it?  The user just types their search terms, so I won't know which element (firstname, surname, companyname) each search term (sun or bat) refers to.</p>