!/bin/bash
 
set -- tomfoolery FoO bar # simulated command line arguments to make it run here
 
#input/output strings
input=$1 #tomfoolery
d=$2 #FOo;
e=$3 #bar;
# turn strings into character-arrays
D=($(echo "$d" |grep -o .))
E=($(echo "$e" |grep -o .))
# define output as empty
o=""
# permutate over the individual characters of 'd' and set the case of the corresponding character in 'e'
for i in $(seq 0 $((${#D[@]}-1)) );do
  printf '%s' "${D[i]} --> "; # for debugging only
  if [[ $(echo "${D[i]}" |grep -c '[A-Z]') -eq 1 ]]; then
    echo "${E[i]}" |tr '[:lower:]' '[:upper:]'; # for debugging only
    o=${o}$(echo "${E[i]}" |tr '[:lower:]' '[:upper:]');
  else
    echo "${E[i]}"; # for debugging only
    o=${o}"${E[i]}";
  fi;
done
echo "$o" # for debugging only
echo "$input" |sed "s@$d@$o@i"
 

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