reverse_case() {
    # Usage: reverse_case "string"
    printf '%s
' "${1~~}"
}

$ reverse_case "hello"
HELLO

$ reverse_case "HeLlO"
hElLo

$ reverse_case "HELLO"
hello

trim_quotes() {
    # Usage: trim_quotes "string"
    : "${1//'}"
    printf '%s
' "${_//"}"
}

$ var="'Hello', "World""
$ trim_quotes "$var"
Hello, World

strip_all() {
    # Usage: strip_all "string" "pattern"
    printf '%s
' "${1//$2}"
}

$ strip_all "The Quick Brown Fox" "[aeiou]"
Th Qck Brwn Fx

$ strip_all "The Quick Brown Fox" "[[:space:]]"
TheQuickBrownFox

$ strip_all "The Quick Brown Fox" "Quick "
The Brown Fox