FYI, you're missing two patterns that allow the `--key=value` admirers and the `-alltheshortopsinasinglestring` spacebar savers among us to be happy (for the otherwise excellent options parsing code).
shopt -s extglob
case "$1"
# Flag support - allow -xyz z-takes-params
-@(a|b|c)*) _flag=${1:1:1}; _rest=${1:2}; shift; set -- "-$_flag" "-$_rest" "$@";;
# Param=Value support
-?(-)*=*) _key=${1%%=*}; _value=${1#*=}; shift; set -- "${_key}" "$_value" "$@";;
esac
