source: vendor/bash/3.1/examples/functions/substr2

#
# substr -- a function to emulate the ancient ksh builtin
#

# -l == remove shortest from left
# -L == remove longest from left
# -r == remove shortest from right (the default)
# -R == remove longest from right

substr()
{
        local flag pat str
        local usage="usage: substr -lLrR pat string or substr string pat"
        local options="l:L:r:R:"

        OPTIND=1
        while getopts "$options" c
        do
                case "$c" in
                l | L | r | R)
                        flag="-$c"
                        pat="$OPTARG"
                        ;;
                '?')
                        echo "$usage"
                        return 1
                        ;;
                esac
        done

        if [ "$OPTIND" -gt 1 ] ; then
                shift $[ $OPTIND -1 ]
        fi

        if [ "$#" -eq 0 ] || [ "$#" -gt 2 ] ; then
                echo "substr: bad argument count"
                return 2
        fi

        str="$1"

        #
        # We don't want -f, but we don't want to turn it back on if
        # we didn't have it already
        #
        case "$-" in
        "*f*")
                ;;
        *)
                fng=1
                set -f
                ;;
        esac

        case "$flag" in
        -l)
                str="${str#$pat}"               # substr -l pat string
                ;;
        -L)
                str="${str##$pat}"              # substr -L pat string
                ;;
        -r)
                str="${str%$pat}"               # substr -r pat string
                ;;
        -R)
                str="${str%%$pat}"              # substr -R pat string
                ;;
        *)
                str="${str%$2}"                 # substr string pat
                ;;
        esac

        echo "$str"

        #
        # If we had file name generation when we started, re-enable it
        #
        if [ "$fng" = "1" ] ; then
                set +f
        fi
}