What does [CFWS] and [FWS] mean in this ABNF?

Question

The RFC 2282 for emails have the below ABNF for quoted-string.

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

I googled and foundthat CFWS is Comments, Folding, Whitespaces. I know what whitespaces are but don't what comments and folding is in terms of ABNF in an email address.

Also what does [FWS] inside *() mean? The double quotes can have 0 or more occurences of qcontent preceded by Folding and whitespaces?

This is very confusing. References to understand ABNF would be much appreciated.

Answer 1

This isn't part of the generic ABNF syntax (currently defined in RFC 5234, although RFC 2234 was the definition of ABNF in play at the time that RFC 2282 was written). Rather, FWS and CFWS are special tokens defined in the email RFC itself (see section 3.2.3 of RFC 2822, or section 3.2.2 of RFC 5322, which obsoleted RFC 2822 in 2008).

From RFC 5322:

2.2.3. Long Header Fields

Each header field is logically a single line of characters comprising the field name, the colon, and the field body. For convenience however, and to deal with the 998/78 character limitations per line, the field body portion of a header field can be split into a multiple-line representation; this is called "folding". The general rule is that wherever this specification allows for folding white space (not simply WSP characters), a CRLF may be inserted before any WSP.

For example, the header field:

Subject: This is a test

can be represented as:

Subject: This
 is a test

...

The process of moving from this folded multiple-line representation of a header field to its single line representation is called "unfolding". Unfolding is accomplished by simply removing any CRLF that is immediately followed by WSP. Each header field should be treated in its unfolded form for further syntactic and semantic evaluation. An unfolded header field has no length restriction and therefore may be indeterminately long.

...

3.2.2. 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 3.2.4. Comments may nest.

There are several places in this specification 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 specification, 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) /  obs-FWS
                                       ; Folding white space

ctext           =   %d33-39 /          ; Printable US-ASCII
                    %d42-91 /          ;  characters not including
                    %d93-126 /         ;  "(", ")", or "\"
                    obs-ctext

ccontent        =   ctext / quoted-pair / comment

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

CFWS            =   (1*([FWS] comment) [FWS]) / FWS

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