is_sql.sql

-- check SQL syntax exactly in your PostgreSQL version
CREATE OR REPLACE FUNCTION is_sql(sql text, is_notice boolean default false)
    returns boolean
    returns null on null input
    parallel unsafe --(ERROR:  cannot start subtransactions during a parallel operation)
    language plpgsql
AS
$$
DECLARE
    exception_sqlstate text;
    exception_message text;
    exception_context text;
BEGIN
    BEGIN
        EXECUTE E'DO $IS_SQL$ BEGIN\nRETURN;\n' || trim(trailing E'; \r\n\t' from sql) || E';\nEND; $IS_SQL$;';
    EXCEPTION WHEN syntax_error THEN
        GET STACKED DIAGNOSTICS
            exception_sqlstate = RETURNED_SQLSTATE,
            exception_message = MESSAGE_TEXT,
            exception_context = PG_EXCEPTION_CONTEXT;
        IF is_notice THEN
            RAISE NOTICE 'exception_sqlstate = %', exception_sqlstate;
            RAISE NOTICE 'exception_context = %', exception_context;
            RAISE NOTICE 'exception_message = %', exception_message;
        END IF;
        RETURN FALSE;
    END;
    RETURN TRUE;
END
$$;

-- TEST
do $$
    begin
        --positive
        assert is_sql('SELECT x');
        --negative
        assert not is_sql('SELECTx', true);
    end;
$$;