example

#include <stdio.h>
#include <ctype.h>

main()
{

    int isIncorrect = 1;

    do {
       char fullTime[6];
       scanf("%s",fullTime);

       // check time format
       if( fullTime[5]!='\0') {
           printf("Too many characters!");
       }
       else if (fullTime[2] != ':') {
           printf("Wrong format!");
       }
       else if( !(isdigit(fullTime[0]) && isdigit(fullTime[1]) && isdigit(fullTime[3]) && isdigit(fullTime[4]) ) ) {
           printf("Not a number!");
       }
       else {
           printf("%s",fullTime);
           isIncorrect=0;
       }
       puts("");
    }
    while( isIncorrect );

    return 0;
}