Awk: Alter lines depending on the next one

Question

How can I use awk to modify lines that match a certain pattern, but only if the NEXT line does not match another pattern?

For example take this fragment:

....
pattern1 100 200 300
pattern1 300 400 400
pattern2 300 900 700
pattern1 200 500 900
...

I would like to modify all lines containing pattern1, except for the second one which is followed by a line containing pattern2.

This is similar to this question, but just the other way around.

Answer 1

try something like

/pattern2/ { print intact ; print $0 ; next ;}
! /pattern2/  { print new ;}
{ intact=$0 ; $1="p1" ; new=$0 ; }

there is an extra empty line

result is

p1 100 200 300
pattern1 300 400 400
pattern2 300 900 700
p1 300 400 400

Answer 2

This does what I needed:

/pattern1/ {
    if (candidate) {
        print "modify:", candidate
    }
    candidate=$0
    next
}

/pattern2/ {
    if (candidate) {
        print candidate
        candidate=0
    }
}

{
    if (candidate) {
        print "modify:", candidate
        candidate=0
    }
    print $0
}

Answer 3

sed -e '
   /pattern1/!b
   $q; N
   /\npattern2/!s/^/MODIFIED:/
   P;D
' yourfile

perl -0777pe 's/(?=pattern1.*$)(?!pattern2)/MODIFIED:/gm' yourfile