ANTLR4 grammar: getting mismatched input error - antlr

Question

I have defined the following grammar:
grammar Test;
parse: expr EOF;
expr : IF comparator FROM field THEN #comparatorExpr
;
dateTime : DATE_TIME;
number : (INT|DECIMAL);
field : FIELD_IDENTIFIER;
op : (GT | GE | LT | LE | EQ);
comparator : op (number|dateTime);
fragment LETTER : [a-zA-Z];
fragment DIGIT : [0-9];
IF : '$IF';
FROM : '$FROM';
THEN : '$THEN';
OR : '$OR';
GT : '>' ;
GE : '>=' ;
LT : '<' ;
LE : '<=' ;
EQ : '=' ;
INT : DIGIT+;
DECIMAL : INT'.'INT;
DATE_TIME : (INT|DECIMAL)('M'|'y'|'d');
FIELD_IDENTIFIER : (LETTER|DIGIT)(LETTER|DIGIT|' ')*;
WS : [ \r\t\u000C\n]+ -> skip;
And I try to parse the following input:
$IF >=15 $FROM AgeInYears $THEN
it gives me the following error:
line 1:6 mismatched input '15 ' expecting {INT, DECIMAL, DATE_TIME}
All SO posts I found point out to the same reason for this error - identical LEXER rules. But I cannot see why 15 can be matched to either DECIMAL - it requires . between 2 ints, or to DATE_TIME - it has m|d|y suffix as well.
Any pointers would be appreciated here.

Answer 1

It's always a good idea to run take a look at the token stream that your Lexer produces:
grun Test parse -tokens -tree Test.txt
[#0,0:2='$IF',<'$IF'>,1:0]
[#1,4:5='>=',<'>='>,1:4]
[#2,6:8='15 ',<FIELD_IDENTIFIER>,1:6]
[#3,9:13='$FROM',<'$FROM'>,1:9]
[#4,15:25='AgeInYears ',<FIELD_IDENTIFIER>,1:15]
[#5,26:30='$THEN',<'$THEN'>,1:26]
[#6,31:30='<EOF>',<EOF>,1:31]
line 1:6 mismatched input '15 ' expecting {INT, DECIMAL, DATE_TIME}
(parse (expr $IF (comparator (op >=) 15 ) $FROM (field AgeInYears ) $THEN) <EOF>)
Here we see that "15 " (1 5 space) has been matched by the FIELD_IDENTIFIER rule. Since that's three input characters long, ANTLR will prefer that Lexer rule to the INT rule that only matches 2 characters.
For this particular input, you can solve this be reworking the FIELD_IDENTIFIER rule to be:
FIELD_IDENTIFIER: (LETTER | DIGIT)+ (' '+ (LETTER | DIGIT))*;
grun Test parse -tokens -tree Test.txt
[#0,0:2='$IF',<'$IF'>,1:0]
[#1,4:5='>=',<'>='>,1:4]
[#2,6:7='15',<INT>,1:6]
[#3,9:13='$FROM',<'$FROM'>,1:9]
[#4,15:24='AgeInYears',<FIELD_IDENTIFIER>,1:15]
[#5,26:30='$THEN',<'$THEN'>,1:26]
[#6,31:30='<EOF>',<EOF>,1:31]
(parse (expr $IF (comparator (op >=) (number 15)) $FROM (field AgeInYears) $THEN) <EOF>)
That said, I suspect that attempting to allow spaces within your FIELD_IDENTIFIER (without some sort of start/stop markers), is likely to be a continuing source of pain as you work on this. (There's a reason why you don't see this is most languages, and it's not that nobody thought it would be handy to allow for multi-word identifiers. It requires a greedy lexer rule that is likely to take precedence over other rules (as it did here)).