Dfyjkt

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.