grammar ::= (name '::=' expression end)*
expression ::= expression-sequence ('|' expression-sequence)*
expression-squence ::= expression-part+
expression-part ::=
  terminal |
  non-terminal |
  special |
  '(' expression ')'

terminal ::=
  "'" (string-character | "\\'")* "'" |
  '"' (string-character | '\\"')* '"'
non-terminal ::= [a-zA-Z0-9\-_]+
special ::= '[' (string-character | '\\]')* ']'

string-character ::= [any utf-8 character] | '\\\\' | '\\.' | '\\-'
end ::= [newline without indentation] | [end of file]