rfc2822 April 2001

3.2.5. Quoted strings

   Strings of characters that include characters other than those
   allowed in atoms may be represented in a quoted string format, where
   the characters are surrounded by quote (DQUOTE, ASCII value 34)
   characters.

qtext           =       NO-WS-CTL /     ; Non white space controls

              %d33 /          ; The rest of the US-ASCII
              %d35-91 /       ;  characters not including "\"
              %d93-126        ;  or the quote character

qcontent        =       qtext / quoted-pair

quoted-string   =       [CFWS]
              DQUOTE *([FWS] qcontent) [FWS] DQUOTE
              [CFWS]

   A quoted-string is treated as a unit.  That is, quoted-string is
   identical to atom, semantically.  Since a quoted-string is allowed to
   contain FWS, folding is permitted.  Also note that since quoted-pair
   is allowed in a quoted-string, the quote and backslash characters may
   appear in a quoted-string so long as they appear as a quoted-pair.

   Semantically, neither the optional CFWS outside of the quote
   characters nor the quote characters themselves are part of the
   quoted-string; the quoted-string is what is contained between the two
   quote characters.  As stated earlier, the "\" in any quoted-pair and
   the CRLF in any FWS/CFWS that appears within the quoted-string are
   semantically "invisible" and therefore not part of the quoted-string
   either.