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

Grep short variant (idea borrowed from sed one):

$ echo -e "String1\nString2" | grep -zoP '^(.*)(?=.*?\n\1)'
String

Assumes string have no new line character. But easy may be tuned to use any delimiter.

Update at 2016-10-24: On modern versions of grep you may receive complain grep: unescaped ^ or $ not supported with -Pz, just use \A instead of ^:

$ echo -e "String1\nString2" | grep -zoP '\A(.*)(?=.*?\n\1)'
String