parsing - can a top-down parser detect ungrammaticality an input string? -

i read possible ,

will require backtracking?

what sketch recovering parsing errors .

the way top down parser can detect ungrammaticality, i.e. invalidity of input string is, example:

if have non-terminal on top of stack instance, , next token in input string symbol b,

then go parse table , go row a, , column b, , if there empty cell, input string invalid.

a method recover entry panic mode, skip tokens in input string until find 1 in synchronising set, , pop off stack , continue.

several ways of choosing synchronising set, follow(a) example