Longest common prefix of two strings in bash

Question

I have two strings. For the sake of the example they are set like this:

string1=\"test toast\"
string2=\"test test\"

What I want is to find

Answer 1

In sed, assuming the strings don't contain any newline characters:

string1="test toast"
string2="test test"
printf "%s\n%s\n" "$string1" "$string2" | sed -e 'N;s/^\(.*\).*\n\1.*$/\1/'