This is an example of a recursive descent RFC 5322 date parser written using Boost.Spirit.

This parser was developed incrementally, commit by commit. Browse the tags to see how this parser evolved from simple cases up to handling the basic RFC 5322 date grammar.

After handling the basics, validation of the input was added.

Finally, parsing the "obsolete" elements of RFC 5322 was added.

Tips

Here are some tips for creating parsers with Spirit:

• Develop your parsers incrementally, starting with the smallest parsable unit. Drive your parser forward step-by-step with unit tests to keep everything working as you enhance the parser.
• When building up parser rules and grammars driven by tests:
  • Use lit("expected input text") and attr() to match the input of test cases and supply the necessary attribute values. As you add more test cases for your grammar, you will drive out these intermediate steps with parser details.
  • Start with the simplest cases that omit all optional elements of your grammar.
  • Start with a single choice among alternatives in your rules. Use additional tests to drive the remaining alternatives into your grammar. This keeps your grammar from adding too many new rules and non-terminals in a single step, keeping it easy to debug.
  • Extract grammar rules incrementally as you build up functionality. Refactor out rules in your grammar after passing new test cases.
• Remember that rule and grammar attribute types are specified by function signatures: write rule<Iter, bool(), skipper> boolean, not rule<Iter, bool, skipper> boolean.
• Order the elements of a struct in BOOST_FUSION_ADAPT_STRUCT so that it is convenient for your parser attribute sequence.
• When using BOOST_FUSION_ADAPT_STRUCT, make sure that your grammar generates attributes for each adapted struct member.
• Use the & parser (and predicate) when you require a prefix to exist in the input stream, but want those characters to be consumed by a subsequent parser. For example, &(lit('+') | '-') >> int_ requires an integer quantity to have a sign prefix, but leaves that prefix in place for int_ to consume so that the attribute value is appropriately signed.
• When you get a strange compile error in a Spirit header, look for a comment in the header that explains the most likely cause of the compile error and what to do about it. The Spirit library authors have tried to anticipate common mistakes to avoid "template compile error hell".
• Spirit parsers leverage templates heavily to achieve fast runtime at the expense of compile time. Isolate your parsers behind an application specific API. The parser implementation only needs to be recompiled when the parser changes. The parser can be reused in as many places as possible without recompiling the parser.

RFC 5322 Date Productions

   date-time       =   [ day-of-week "," ] date time [CFWS]

   day-of-week     =   ([FWS] day-name) / obs-day-of-week

   day-name        =   "Mon" / "Tue" / "Wed" / "Thu" /
                       "Fri" / "Sat" / "Sun"

   date            =   day month year

   day             =   ([FWS] 1*2DIGIT FWS) / obs-day

   DIGIT           =   %x30-39  ; 0-9

   month           =   "Jan" / "Feb" / "Mar" / "Apr" /
                       "May" / "Jun" / "Jul" / "Aug" /
                       "Sep" / "Oct" / "Nov" / "Dec"

   year            =   (FWS 4*DIGIT FWS) / obs-year

   time            =   time-of-day zone

   time-of-day     =   hour ":" minute [ ":" second ]

   hour            =   2DIGIT / obs-hour

   minute          =   2DIGIT / obs-minute

   second          =   2DIGIT / obs-second

   zone            =   (FWS ( "+" / "-" ) 4DIGIT) / obs-zone

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

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

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

   ccontent        =   ctext / quoted-pair / comment

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

   quoted-pair     =   ("\" (VCHAR / WSP)) / obs-qp

   VCHAR           =   %x21-7E
                       ; visible (printing) characters

   WSP             =   SP / HTAB
                       ; white space

   SP              =   %x20

   HTAB            =   %x09     ; horizontal tab

   CRLF            =   CR LF    ; Internet standard newline

   LF              =   %x0A     ; linefeed

   CR              =   %x0D     ; carriage return

   obs-day-of-week =   [CFWS] day-name [CFWS]

   obs-day         =   [CFWS] 1*2DIGIT [CFWS]

   obs-year        =   [CFWS] 2*DIGIT [CFWS]

   obs-hour        =   [CFWS] 2DIGIT [CFWS]

   obs-minute      =   [CFWS] 2DIGIT [CFWS]

   obs-second      =   [CFWS] 2DIGIT [CFWS]

   obs-zone        =   "UT" / "GMT" /     ; Universal Time
                                          ; North American UT
                                          ; offsets
                       "EST" / "EDT" /    ; Eastern:  - 5/ - 4
                       "CST" / "CDT" /    ; Central:  - 6/ - 5
                       "MST" / "MDT" /    ; Mountain: - 7/ - 6
                       "PST" / "PDT" /    ; Pacific:  - 8/ - 7
                                          ;

   obs-ctext       =   obs-NO-WS-CTL

   obs-NO-WS-CTL   =   %d1-8 /            ; US-ASCII control
                       %d11 /             ;  characters that do not
                       %d12 /             ;  include the carriage
                       %d14-31 /          ;  return, line feed, and
                       %d127              ;  white space characters

   obs-qp          =   "\" (%d0 / obs-NO-WS-CTL / LF / CR)

   obs-FWS         =   1*WSP *(CRLF 1*WSP)

This repository contains source code to accompany a Utah C++ Programmers presentation.

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