As regular expression has equivalent automaton i.e., the finite automaton, similarly all context free grammars have an equivalent machine i.e., pushdown automaton. The pushdown automation is essentially a finite automaton with control of both an input take and a stack or â€œfirst-in-last-outâ€ list. A symbol may be entered or removed only at the top of the list. When a symbol is entered at the top, the symbol previously at the top becomes second from the top, the symbol previously second from the top becomes third and so on. Similarly, when a symbol is removed from the top of the list, the symbol previously second from the top becomes the top symbol the symbol previously third from the top becomes the second and so on.