rfc2822 April 2001

3.2.3. Folding white space and comments

   White space characters, including white space used in folding
   (described in section 2.2.3), may appear between many elements in
   header field bodies.  Also, strings of characters that are treated as
   comments may be included in structured field bodies as characters
   enclosed in parentheses.  The following defines the folding white
   space (FWS) and comment constructs.

   Strings of characters enclosed in parentheses are considered comments
   so long as they do not appear within a "quoted-string", as defined in
   section 3.2.5.  Comments may nest.

   There are several places in this standard where comments and FWS may
   be freely inserted.  To accommodate that syntax, an additional token
   for "CFWS" is defined for places where comments and/or FWS can occur.
   However, where CFWS occurs in this standard, it MUST NOT be inserted
   in such a way that any line of a folded header field is made up
   entirely of WSP characters and nothing else.

FWS             =       ([*WSP CRLF] 1*WSP) /   ; Folding white space
              obs-FWS

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

              %d33-39 /       ; The rest of the US-ASCII
              %d42-91 /       ;  characters not including "(",
              %d93-126        ;  ")", or "\"

ccontent        =       ctext / quoted-pair / comment

comment         =       "(" *([FWS] ccontent) [FWS] ")"

CFWS            =       *([FWS] comment) (([FWS] comment) / FWS)

   Throughout this standard, where FWS (the folding white space token)
   appears, it indicates a place where header folding, as discussed in
   section 2.2.3, may take place.  Wherever header folding appears in a
   message (that is, a header field body containing a CRLF followed by
   any WSP), header unfolding (removal of the CRLF) is performed before
   any further lexical analysis is performed on that header field
   according to this standard.  That is to say, any CRLF that appears in
   FWS is semantically "invisible."

   A comment is normally used in a structured field body to provide some
   human readable informational text.  Since a comment is allowed to
   contain FWS, folding is permitted within the comment.  Also note that
   since quoted-pair is allowed in a comment, the parentheses and
   backslash characters may appear in a comment so long as they appear
   as a quoted-pair.  Semantically, the enclosing parentheses are not
   part of the comment; the comment is what is contained between the two
   parentheses.  As stated earlier, the "\" in any quoted-pair and the
   CRLF in any FWS that appears within the comment are semantically
   "invisible" and therefore not part of the comment either.

   Runs of FWS, comment or CFWS that occur between lexical tokens in a
   structured field header are semantically interpreted as a single
   space character.