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

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

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

substr()
{
        local flag pat str
        local usage="usage: substr -lLrR pat string or substr string pat"

        case "$1" in
        -l | -L | -r | -R)
                flag="$1"
                pat="$2"
                shift 2
                ;;
        -*)
                echo "substr: unknown option: $1"
                echo "$usage"
                return 1
                ;;
        *)
                flag="-r"
                pat="$2"
                ;;
        esac

        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
}