import java.util.Set;
import java.util.TreeSet;

public class LineCharLimiter {

    private final Set<Character> chars = new TreeSet<>();


    public LineCharLimiter(String chars){
        for(char ch : chars.toCharArray()){
            this.chars.add(ch);
        }
    }


    public String handle(String str){
        for(char ch : str.toCharArray()){
            if(!this.chars.contains(ch))
                return null;
        }

        return str;
    }
}