I'm trying to implement a check_host function based on the SPF RFC7208. It is almost ready and the thing that has left is the macro expansion detailed in section 7 (https://www.rfc-editor.org/rfc/rfc7208#section-7). I know there are readily available solutions to this but for the sake of example or practice I want to implement my own algorithm. While doing so I have the problem that www.example.com does not fit the ABNF description. I assume it is because of my wrong reasoning and that is why I'm asking for help.
This is the ABNF copied from the document:
domain-spec = macro-string domain-end
domain-end = ( "." toplabel [ "." ] ) / macro-expand
toplabel = ( *alphanum ALPHA *alphanum ) /
( 1*alphanum "-" *( alphanum / "-" ) alphanum )
alphanum = ALPHA / DIGIT
explain-string = *( macro-string / SP )
macro-string = *( macro-expand / macro-literal )
macro-expand = ( "%{" macro-letter transformers *delimiter "}" )
/ "%%" / "%_" / "%-"
macro-literal = %x21-24 / %x26-7E
; visible characters except "%"
macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
"c" / "r" / "t" / "v"
transformers = *DIGIT [ "r" ]
delimiter = "." / "-" / "+" / "," / "/" / "_" / "="
I think that applying domain-spec to www.example.com would follow this path: domain-spec -> macro-string -> macro-literal (repeated) and this will eat up the whole string and then domain-end will never match.
Where am I wrong?
Edit 1:
I think I got the answer and the original question becomes more like a request for a confirmation. The repetition in ABNF is greedy but probably allows for back off - i.e. eat less if this would lead to a match. Looking at RFC5234 (https://www.rfc-editor.org/rfc/rfc5234), though not in its entirety, I was not able to definitely spot this. Can anyone confirm?