antlr string parser rule takes precedent over other rules - antlr

Question

I have the following grammar:
cell
: operator value
;
operator
: EQ
;
value
: StringCharacters
;
EQ
: '='
;
StringCharacters
: StringCharacter+
;
fragment
StringCharacter
: ~[\\\r\n]
;
WS : [ \t\r\n\u000C]+ -> skip
;
The idea is to allow the following inputs:
= 3
=3
=asdkfljer
=skdfj wkrje slkjf
and so on and have the parser recognize the preceding operator all the time. But that's exactly not what is happening. Instead, the parser always recognizes everything as a value.
How can I implement the grammar in such a way that the parser always recognizes the operator first and basically accepts the rest as the value?

Answer 1

The problem is that StringCharacters matches any of your input string and ANTLR takes a token with the greatest length possible.
To solve this I'd suggest using Lexical Modes, something like:
EQ
: '=' -> pushMode(VALUE_MODE)
;
mode VALUE_MODE;
StringCharacters
: StringCharacter+ -> popMode
;
fragment
StringCharacter
: ~[\\\r\n]
;
WS : [ \t\r\n\u000C]+ -> skip
;
Note, the example above will be able to parse only one line.
If you want to parse multiple lines of values, you have to modify the lexer and the parser:
Lexer:
EQ
: '=' -> pushMode(VALUE_MODE)
;
mode VALUE_MODE;
StringCharacters
: StringCharacter+ [\r\n]* -> popMode
;
fragment
StringCharacter
: ~[\\\r\n]
;
WS : [ \t\r\n\u000C]+ -> skip
;
Parser:
cell
: (operator value)*
;
operator
: EQ
;
value
: StringCharacters
;