Use the drop down to select the number of letters. Then enter either a single character or a period (.) to indicate a missing letter in each box.
For example, 'k', 'i', 'i', '.' to search for a four-letter word where three of
the letters are a 'k' and two 'i's, and the fourth is unknown.
Number of letters
Word list from https://github.com/dwyl/english-words.
The C source for the code to do the search is available