%{
#include <stdio.h>
 
int yylex(void);
void yyerror(const char *s);
%}
 
%token A B C
 
%%
 
S : A S B
  | B S A
  | C
  ;
 
%%
 
int yylex()
{
    int ch;
 
    ch = getchar();
 
    if (ch == 'a')
        return A;
    if (ch == 'b')
        return B;
    if (ch == 'c')
        return C;
 
    return 0;
}
 
void yyerror(const char *s)
{
    printf("Invalid string\n");
}
 
int main()
{
    printf("Enter string: ");
 
    if (yyparse() == 0)
        printf("Valid string\n");
 
    return 0;
}
 

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