Suppose for example we have a couple of wordlists words_foo, words_bar and we want to find a c/u pair where each word might be in either list (or both). Assuming that all the words are strings of lower-case letters, we can find all desired pairs with the Unix command
cat words_foo words_bar | sort | uniq | grep '\<[cu]' | sed 's/./?/' | sort | uniq -c | grep 2
If there's one wordlist words then the initial “cat words | sort | uniq” can be simplified to “sort words | sort | uniq”, and indeed the “ | sort | uniq” is not needed if the words are already in alphabetical order without repetition, as is often the case. The output will depend on how extensive your wordlist(s) might be; the first one I tried this on contained only the canonical solution

   2 ?ranium