BOF (Begin of File) token for elements that must appear at the beginning of a line

Hey, this is not to report an issue but to request a feature, so some items in the checklist do not apply I believe.

This is to request a “BOF”(Begin of File) token in the Lexer. I understand that there is a closed issue #1016 on this topic. The example provided in that issue did not really need a BOF token. But I believe a BOF token will benefit in my situation.

Sample grammar:

grammar com;

comment: COMMENT;

COMMENT
 : '\n' '//' .*? '\n'
 ;

By design, it will match \n//comment\n but not abcd //comment\n. But I also want it to match //comment\n when it is at the beginning of the file. A BOF token will allows me to upgrade the lexer to COMMENT: [\n<BOF>] '//' .*? '\n'.

I did ask a related question on Stackoverflow. The question was seeking a solution. This is to suggest a feature because I believe BOF does not exist now.

[KvanTTT]

Try to read https://github.com/antlr/antlr4/blob/master/doc/predicates.md and just use JS code instead of Java in semantic predicates.

[MrCoder]

You can achieve such behavior using semantic predicate, but yes, it’s not language agnostic way.

Because Antlr4 is targeting multiple languages, it is a reasonable feature to be added. More and more people I assume will use it in JavaScript/TypeScript.