用多个字符替换字符串中的两个字符？

So I'm having trouble with this problem I'm trying to solve. Let's say I have a user input a sequence we want to find in a string. But they're allowed to enter 'X' where it will be replaced by 4 characters and search the string. For example if the user enters AXXT, it would search the string for AGCT, AGGT, etc. So this is what I tried to do:So I'm having trouble with this problem I'm try