shell replace or write new string code example

Example 1: bash replace substring

echo [string] | sed "s/[original]/[target]/g"

Example 2: bash replace beginning of string

$ cat shortest.sh
#! /bin/bash

filename="bash.string.txt"

echo ${filename#*.}
echo ${filename%.*}

$ ./shortest.sh
After deletion of shortest match from front: string.txt
After deletion of shortest match from back: bash.string