Use the drop down to select the number of letters. Then enter either a single character or the number in the range 1 to 27 of the searched for letter in each input box.
For example, '1', '2', 'w', '2' to search for a four-letter word where the
third letter is w, the first character is different to the other letters and
the same letter is repeated in the 2nd and 4th character positions.
Number of letters
Word list from https://github.com/dwyl/english-words.
The C source for the code to do the search is available