Question : Unix update variable by position

Hi
Im new to Unix, as part of my learning im creating a hangman game using bash. I am having a problem showing what letters a user has guessed correctly.
So far I have 4 variables $word - that is the word a user is trying to guess, $hidden - that is the same as $word but replaces each letter with a -,  $input_letter that is the current letter guess, and $position which is where the letter occurs in word.
eg $word = join
$hidden = ----
$input_letter = i
$position = 3

I want to replace 1 character in $hidden with $input_letter at $position. I also have to do this without the use of sed or awk.

Any help much appreciated

(unsure of which zone this would come under)

Answer : Unix update variable by position

#!/bin/bash
word=join
input_letter=i
hidden=${word//[^$input_letter]/-}
echo $hidden