Replace any four alphanumeric characters followed by an @ symbol, two strings of alphanumeric characters separated by a period, two lowercase letters, and a forward slash with an ampersand in the file.txt.
sed -i 's/[[:alnum:]]\{4\}\@\([[:alnum:]]\+\.\)\{2\}[[:upper:]][[:lower:]]\{2\}\/\&/g' file.txt