Replace & sign to &amp word

Shell Programming and Scripting
1

Hi, I have text file abc.txt. In this file, I have the following data.

Input:

Mr Smith &amp Mrs Smith
Mr Smith &apos Mrs Smith
Mr Smith & Mrs Smith
Mr Smith& Mrs Smith
Mr Smith &Mrs Smith

Output:

Mr Smith &amp Mrs Smith
Mr Smith &apos Mrs Smith
Mr Smith &amp Mrs Smith
Mr Smith&amp Mrs Smith
Mr Smith &ampMrs Smith

In line 3, 4, and 5. I want to change & sign to &amp word but do not want to change anything in line 1, and line 2. If I use

sed 's/&/&amp/g' abc.txt

command, it changes to &amp everywhere.

Thanks for your help

Naveed

2 
sed 's/&\([^a-z]\)/\&amp\1/g' abc.txt
3

Thanks a lot jlliagre. It works great.

Naveed

4 
sed '2,$s/&/&aa/'
5

Huh ?
Your code give that output:

Mr Smith &amp Mrs Smith
Mr Smith &aaapos Mrs Smith
Mr Smith &aa Mrs Smith
Mr Smith&aa Mrs Smith
Mr Smith &aaMrs Smith