Question

I want to parse elements of RFC822 (SMTP) "Received" lines, which are defined formally in the spec, e.g.:

atom        =  1*

[...]

received    =  "Received"    ":"            ; one per relay
                       ["from" domain]           ; sending host
                       ["by"   domain]           ; receiving host
                       ["via"  atom]             ; physical path
                      *("with" atom)             ; link/mail protocol
                       ["id"   msg-id]           ; receiver msg id
                       ["for"  addr-spec]        ; initial form
                       ";"    date-time         ; time received

[...]

msg-id      =  ""            ; Unique message id

[...]

addr-spec   =  local-part "@" domain        ; global address

etc. for domain, date-time, etc.

Here’s a real example:

Received: from ll-194.132.162.89.kv.sovam.net.ua (ll-194.132.162.89.kv.sovam.net.ua [83.170.243.194] (may be forged)) by raq2073.uk2.net (8.10.2/8.10.2) with ESMTP id lASHDDE10765 for <[e-mail]>; Wed, 28 Nov 2007 17:13:13 GMT

Would regex be a good strategy to capture the parts of a received line?

I realize that many SMTP servers don’t format received lines properly (in real life).

Otherwise, does anyone know of a library in Java that does this well?

Edit Here’s a fiddle showing a regex and tests that I’ve banged on for a while, which seems to work.

Received:\s+(?:from\s+(.+?))?(?:\(qmail (.+?)\))?(?:\s+by\s+(.+?))?(?:\\s+via\s+(.+?))?(?:\s+with\s+(.+?))?(?:\;?\s+id\s+(.+?))?(?:\s+for\s+(.+?))?(?:;\s*(?!.*\;.*)(.+))?$

Answer 1

The choice really depends on exactly what you want to achieve.

For capturing specific parts of a Receiver-line (e.g. ‘give me the From-part’), regexes are awesome.

If you need a full-fledged parser for this grammar, then regexes alone will not suffice. Especially the addr-spec has so many special cases that a regex cannot hope to handle each one correctly (explanation). Regexes are not parsers.

Last time I needed an actual parser, I wrote my own using JavaCC. I would only recommend going down that road if you know a thing or two about grammars and parsing.