ANTLR4 tells me: mismatched input 'little' expecting {'big', 'little'} - antlr4

Question

I have the following simple grammar:
grammar TestG;
p : pDecl+ ;
pDecl : endianDecl
| dTDecl
;
endianType : E_BIG
| E_LITTLE
;
endianDecl : 'endian' '=' endianType ';' ;
dTDecl : 'dT' '[' STRING ']' '=' ID ';' ;
STRING: '"'.*?'"' ; //Embedded quotes?
COMMENT: '#' .*? [\n\r] -> skip ; // Discard comments for now
ID : [a-zA-Z][a-zA-Z0-9_]* ;
WS : [ \t\n\r]+ -> skip ;
INT : ('0x')?[0-9]+ ; // How to handle 0xDD and ensure non zero?
E_BIG : 'big' ;
E_LITTLE : 'little' ;
When I run grun TestG p and input the following:
endian = little;
I get the following:
line 1:9 mismatched input 'little' expecting {'big', 'little'}
What have I done wrong?

Answer 1

Because your lexer rule for ID precedes that for E_LITTLE, your 'little' input is being lexed as an ID.
[#0,0:5='endian',<'endian'>,1:0]
[#1,7:7='=',<'='>,1:7]
[#2,9:14='little',<ID>,1:9] <== see here it's being lexed as an ID
[#3,15:15=';',<';'>,1:15]
[#4,18:17='<EOF>',<EOF>,2:0]
line 1:9 mismatched input 'little' expecting {'big', 'little'}
Moving the these lexer tokens above ID like so:
STRING: '"'.*?'"' ; //Embedded quotes?
COMMENT: '#' .*? [\n\r] -> skip ; // Discard comments for now
E_BIG : 'big' ;
E_LITTLE : 'little' ;
ID : [a-zA-Z][a-zA-Z0-9_]* ;
WS : [ \t\n\r]+ -> skip ;
INT : ('0x')?[0-9]+ ; // How to handle 0xDD and ensure non zero?
yields the correct output from your test input.
[#0,0:5='endian',<'endian'>,1:0]
[#1,7:7='=',<'='>,1:7]
[#2,9:14='little',<'little'>,1:9] <== see here being lexed correctly
[#3,15:15=';',<';'>,1:15]
[#4,18:17='<EOF>',<EOF>,2:0]
Remember, for lexer tokens, the longest match wins, but in the case of a tie, the one that appears FIRST wins. This is why you want your more specific lexer tokens at the top of the lexer token list, and the more general ones (like identifiers, strings, etc.) farther down.