Non case sensitive search case sensitive replace

Question

How do I search/and replace like this using sed or another tool

 FOO => BAR
 Foo => Bar
 foo => bar

That is: Look for foo regardless of case, and replace with the same case pattern.

One "solution" is to run sed once for each case-sensitive pattern but it becomes more work.

Answer 1

#!/bin/bash

S="NewYorkCity"         # search string
R="minneapolis"         # replace template (in lowercase)
declare -u T            # -u: automatic convert content to upper case
for ((I=0; I<${#S}; I++)); do    # loop over each single letter of $S
  if [[ "${S:$I:1}" =~ [A-Z] ]]; then   # upper case?
    T="${R:$I:1}"       # copy single letter to $T (convert to upper case)
    echo -n "$T"        # print single upper case letter without linefeed
  else
    echo -n "${R:$I:1}" # print single letter unaltered without linefeed
  fi
done

Output:

MinNeapOlis

---

Same code, more compact:

A="NewYorkCity"        # search
B="minneapolis"        # replace (must be in lowercase)
declare -u X
for ((I=0; I<${#A}; I++)); do if [[ "${A:$I:1}" =~ [A-Z] ]]; then X="${B:$I:1}"; echo -n "$X"; else echo -n "${B:$I:1}"; fi; done