Shell Programming and Scripting

use this code where you are searching for the patter without xyz

sed  -n '
/[ ]a&b&c$/ !{
h
}
/[ ]a&b&c$/ {
H
n
H
x
p
} ' input.txt

#!/usr/bin/env
data=open("file").read().split("\n\n")
for i in data:
    if not "(xyz)" in i:
        print i

# ./test.py
abc(){
when : a&b&c
}

Upon request from Mr. "rakeshawasthi" and to let the benefit to be spreaded between us I will explain the sed command I had wrote earlier:-

code:
sed -n '
/[ ]a&b&c$/ !{
h      # put the non-matching line in the hold buffer
}
/[ ]a&b&c$/ {
H            # found a line that matches
              # append it to the hold buffer

n             # the hold buffer contains 2 lines
	       # get the next line

H             # and add it to the hold buffer

x              # now print it back to the pattern space

p              # and print it.

} ' input.txt

$/="\n\n";
while(<DATA>){
	next if /abc.*\(xyz\).*/s;
	print;
}
__DATA__
abc() {
when : (xyz)&a&b&a
}

abc(){
when : a&b&c
}

abc() {
when : a&b&c&(xyz)
}

 sed '/abc()/{$!N;/\n.*(xyz)/{$!N;d;}}' filename