/// Keyword
token Token='token' Start='start' Right='right' Skip='skip';
/// Punctuator
token Colon=':' Semi=';' Equal='=' LPar='(' RPar=')' LBrak='[' RBrak=']'
      Or='|' Star='*' Plus='+';
/// Identifier for rules and tokens
token Id='<identifier>' Str='<string literal>';
token Predicate='<semantic predicate>' Action='<semantic action>' Binding='<binding>';
token OpenNode='<open node mark>' CloseNode='<close node mark>';
token Comment DocComment Whitespace;

start file;
skip Comment DocComment Whitespace;

file: decl*;
decl:
  token_list
| rule_decl
| start_decl
| right_decl
| skip_decl
;
start_decl: 'start' Id ';';
right_decl: 'right' (Id | Str)+ ';';
skip_decl: 'skip' (Id | Str)+ ';';
token_list: 'token' token_decl+ ';';
token_decl: Id ['=' Str];
rule_decl: Id ':' [regex] ';';

regex: alternation;
alternation: concat ('|' concat)*;
concat: postfix postfix*;
postfix:
  postfix ('*' | '+')
| atomic
| paren
| optional
;
paren: '(' regex ')';
optional: '[' regex ']';
atomic:
  Id
| Str
| Predicate
| Action
| Binding
| OpenNode
| CloseNode
;