Ndtyjky

I want to parse query expressions that look like this:

Person Name=%John%

(Person Name=John% and Address=%Ontario%)

Person Fullname_3="John C. Smith"

But I'm totally new to Antlr4 and can't even figure out how to parse one single TABLE FIELD=QUERY clause. When I run the grammar below in Go as target, I get

line 1:7 mismatched input 'Name' expecting {'not', '(', FIELDNAME} 

for a simple query like

Person Name=John

Why can't the Grammar parse FIELDNAME via parsing fieldsearch->field EQ searchterm->FIELDNAME?

I guess I'm misunderstanding something very fundamental here about how Antlr Grammars work, but what?

/* ANTLR Grammar for Minidb Query Language */



grammar Mdb;



start : searchclause EOF ;



searchclause

    : table expr

    ;



expr

    : fieldsearch

    | unop fieldsearch

    | LPAREN expr relop expr RPAREN

    ;



unop

    : NOT

    ;



relop

    : AND

    | OR

    ;



fieldsearch

    : field EQ searchterm

    ;



field

    : FIELDNAME

    ;



table

    : TABLENAME

    ;



searchterm

    : STRING

    ;



AND

    : 'and'

    ;



OR

    : 'or'

    ;



NOT

    : 'not'

    ;

EQ

    : '='

    ;



LPAREN

    : '('

    ;



RPAREN

    : ')'

    ;



fragment VALID_ID_START

    : ('a' .. 'z') | ('A' .. 'Z') | '_'

    ;



fragment VALID_ID_CHAR

    : VALID_ID_START | ('0' .. '9')

    ;



TABLENAME

    : VALID_ID_START VALID_ID_CHAR*

    ;



FIELDNAME

    : VALID_ID_START VALID_ID_CHAR*

    ;



STRING: '"' ~('n'|'"')* ('"' | { panic("syntax-error - unterminated string literal") } ) ;



WS

   : [ rnt] + -> skip

;

Try looking at the tokens produced for that input using grun Mdb tokens -tokens. It will tell you that the input consists of two table names, an equals sign and then another table name. To match your grammar it would have needed to be a table name, a field name, an equals sign and a string.

The first problem is that TABLENAME and FIELDNAME have the exact same definition. In cases where two lexer rules would produce a match of the same length on the current input, ANTLR prefers the one that comes first in the grammar. So it will never produce a FIELDNAME token. To fix that just replace both of those rules with a single ID rule. If you want to, you can then introduce parser rules tableName : ID ; and fieldName : ID ; if you want to keep the names.

The other problem is more straight forward: John simply does not match your rules for a string since it's not in quotes. If you do want to allow John as a valid search term, you might want to define it as searchterm : STRING | ID ; instead of only allowing STRINGs.