String constants are either  string  literals  or  named  constants.   A
  string  literal  is  a series of characters enclosed in double quotation
  marks ("text") or single quotation marks ('text').  You can embed double
  quotation  marks within single quotation marks ('"text"') and vice versa
  ("'text'").  Note, however, that  the  outer  quotation  marks  must  be
  paired.