minidb | a simple wraper for local/session storage DOM API management | User Interface library

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.