EBNF formal gramar

[clach04]

Spun out of #5

• https://github.com/too-much-todotxt/legacy_todo.txt/tree/todo.txt_PR83_EBNF_formal_gramar
• Add formal grammar definition of format todotxt/todo.txt#83

Copy/paste:

TODO.txt Formal Grammar

The Grammar for a TODO.txt file is provided in Extended Backus-Naur Form (EBNF).
The grammar below uses the conventions defined in the W3C specification for EBNF.

The grammar below assumes that the digits and whitespace classes have been defined with their obvious definitions. The printable class includes all printable characters.

context ::= "@" printable+ ;
project ::= "+" printable+ ;
hashTag ::= "#" printable+ ;
kvpair ::= ( [^@+#:] printable* ) ":" printable+ ;
word ::= ( [^@+#] printable* ) | "@" | "+" | "#" ;

token ::= context | project | hashTag | kvpair | word ;

day ::= 2 * digits ;
month ::= 2 * digits ;
year ::= 4 * digits ;
date ::= year "-" month "-" day ;

createdDate ::= date ;
completedDate ::= date ;

priorityClass ::= [A-Z] ;
priority ::= "(" priorityClass ")" ;

space ::= whitespace + ;

completedMark ::= "x" ;

todoLine ::= ( completedMark space )?
             ( priority space )?
             ( ( completedDate space createdDate space ) | ( createdDate space ) )?
             word+
             whitespace* ;

todoFile ::= whitespace* todoLine* ;