How to use the reserved words inside the string in ANTLR4? - antlr4

Question

I am a newbie to ANTLR4 and language compilers. I am working on building a language compiler using ANTLR4 Java. I have a small problem with parsing strings. The reserved words/ Tokens are getting matched instead of string. For eg: IF is a keyword token in my lexer but how to use "if" as a string?
Lexer file:
lexer grammar testgrammar;
IF : I F;
ENDIF : E N D I F;
ELSE : E L S E;
CASE : C A S E;
ENDCASE : E N D C A S E;
BREAK : B R E A K;
SWITCH : S W I T C H;
SUBSTRING : S U B S T R I N G;
COMMA : ',' ;
SEMI : ';' ;
COLON : ':' ;
LPAREN : '(' ;
RPAREN : ')' ;
DOT : '.' ;// ('.' {$setType(DOTDOT);})? ;
LCURLY : '{' ;
RCURLY : '}' ;
AND : '&&' ;
OR : '||' ;
DOUBLEQUOTES : '"' ;
COMPARATOR : '=='| '>=' | '>' | '<' | '<=' | '!=' ;
SYMBOLS : '§' | '$' | '%' | '/' | '=' | '?' | '#' | '_' | '#' | '€';
LETTER : [A-Za-z\u00e4\u00c4\u00d6\u00f6\u00dc\u00fc\u00df];
NUMERICVALUE : NUMBER ('.' NUMBER)?;
STRING_LITERAL : '\'' ('\'\'' | ~('\''))* '\'';
NOTCONDITION : NOT;
OPERATORS : OPERATOR;
COMMENT : (('/*' .*? '*/') | ('//' ~[\r\n]*)) -> skip;
WS : (' ' | '\t' | '\r' | '\n')+ -> skip;
fragment A:('a'|'A');
fragment B:('b'|'B');
fragment C:('c'|'C');
fragment D:('d'|'D');
fragment E:('e'|'E');
fragment F:('f'|'F');
fragment G:('g'|'G');
fragment H:('h'|'H');
fragment I:('i'|'I');
fragment J:('j'|'J');
fragment K:('k'|'K');
fragment L:('l'|'L');
fragment M:('m'|'M');
fragment N:('n'|'N');
fragment O:('o'|'O');
fragment P:('p'|'P');
fragment Q:('q'|'Q');
fragment R:('r'|'R');
fragment S:('s'|'S');
fragment T:('t'|'T');
fragment U:('u'|'U');
fragment V:('v'|'V');
fragment W:('w'|'W');
fragment X:('x'|'X');
fragment Y:('y'|'Y');
fragment Z:('z'|'Z');
fragment NUMBER:[0-9]+;
fragment OPERATOR: ('+'|'-'|'&'|'*'|'~');
fragment NOT: ('!');
grammar:
parser grammar testParser;
symbolCharacters: (SYMBOLS | operators) ;
word:
( symbolCharacters | LETTER )+
;
wordList:
word+
;
I am not supposed share full grammar. But i have shared enough information i guess. I can understand that the words are formed from LETTERS and Symbol characters. One workaround i can do is making word rule like:
word:
( symbolCharacters | LETTER | IF | SWITCH | CASE | ELSE | BREAK )+
;
I have a lot of tokens. I dont want to add everything individually. Is there any other nice way to accomplish this?
Valid expression
Error expression
How to make the parser ignore the keywords inside the string?

Answer 1

Your same grammar does not have the problem you describe:
➜ antlr4 testgrammar.g4
➜ javac *.java
➜ echo "if 'if' endif" | grun testgrammar tokens -tokens
[#0,0:1='if',<IF>,1:0]
[#1,3:6=''if'',<STRING_LITERAL>,1:3]
[#2,8:12='endif',<ENDIF>,1:8]
[#3,14:13='<EOF>',<EOF>,2:0]
(perhaps you have inadvertently "corrected" the problem as you trimmed your grammar down, so I'll elaborate a bit.)
In short, during the lexing/tokenization phase of ANTLR parsing your input, ANTLR will, naturally, attempt to match you Lexer rules. If ANTLR finds a match of multiple rules for the current characters of your input stream, it follows two rules to determine a "winner".
If a rule matches a longer sequence of input characters, then that rule will be used.
If two rules match the same number of input characters, then the rule appearing first in your grammar will be used.
In your case, neither really comes into play as the grammar, when it reaches the ', will attempt to complete the STRING_LITERAL rule, and will find a match for the characters 'if'. It will never even attempt to match you IF lexer rule.
BTW, I did have to correct the symbolCharacters parser rule to be
symbolCharacters: (SYMBOLS | OPERATORS);