Match most specific rule - antlr4

Question

In my grammar, I want to have both "variable identifiers" and "function identifiers". Essentially, I want to be less restrictive on the characters allowed in function identifiers. However, I am running in to the issue that all variable identifiers are valid function identifiers.
As an example, say I want to allow uppercase letters in a function identifier but not in a variable identifier. My current (presumably naive) might look like:
prog : 'func' FunctionId
| 'var' VariableId
;
FunctionId : [a-zA-Z]+ ;
VariableId : [a-z]+ ;
With the above rules, var hello fails to parse. If I understand correctly, this is because FunctionId is defined first, so "hello" is treated as a FunctionId.
Can I make antlr choose the more specific valid rule?

Answer 1

An explanation why your grammar does not work as expected could be found here.
You can solve this with semantic predicates:
grammar Test;
prog : 'func' functionId
| 'var' variableId
;
functionId : Id;
variableId : {isVariableId(getCurrentToken().getText())}? Id ;
Id : [a-zA-Z]+;
On the lexer level there will be only ids. On the parser level you can restrict an id to lowercase characters. isVariableId(String) would look like:
public boolean isVariableId(String text) {
return text.matches("[a-z]+");
}

Answer 2

Can I make antlr choose the more specific valid rule?
No (as already mentioned). The lexer merely matches as much as it can, and in case 2 or more rules match the same, the one defined first "wins". There is no way around this.
I'd go for something like this:
prog : 'func' functionId
| 'var' variableId
;
functionId : LowerCaseId | UpperCaseId ;
variableId : LowerCaseId ;
LowerCaseId : [a-z]+ ;
UpperCaseId : [A-Z] [a-zA-Z]* ;