How to set STDIN string to specif characters?

Question

How to set input string array to accept only specific letters from STDIN??

char arr[testcases][100];
    for(i=0;i<testcases;i++){
    scanf("%99s",&arr[i]);
}

I am going to store only letters a,b,c,d in my array. How can I restrict other letters from getting stored in my array?

Answer 1

You can use this:

 scanf("%99[a-d]", arr);

The return value will be 1 if any initial part of the string matches; input will stop at the first non-matching character.