Using sed, add line X lines above match

Question

I know that to prepend/append something, I just have to do:

sed -i '/pattern/a new thing!'

However, I'd like to match a line, and prepend something two lines higher.

I'd like to limit tools only to those which come out of the box with ubuntu 12.04, I don't know if awk fits the bill.

Just in case I'm pursuing solution to problem B while I need solution to A: Excerpt of .m3u8 file looks like this:

#EXTINF:4.304300,
output-000.ts
#EXTINF:5.705700,
output-001.ts
#EXTINF:4.304300,
output-002.ts
#EXTINF:5.705700,
output-003.ts

Now, my goal is to insert a line 2 lines before every X'th output-XXX.ts. For example I want to achieve this:

#Inserted line 0
#EXTINF:4.304300,
output-000.ts
#EXTINF:5.705700,
output-001.ts
#Inserted line 1
#EXTINF:4.304300,
output-002.ts
#EXTINF:5.705700,
output-003.ts

Answer 1

With awk (installed by default on POSIX systems including ubuntu):

POSIXLY_CORRECT=1 awk -v X=2 '
  NR >= 2 && /^output-[0-9]{3}\.ts$/ && ++n % X == 1 {
    print "#Inserted line", i++
  }
  NR>1 {print last}
  {last = $0}
  END{if (NR) print last}' < file.in > file.out

That's for 2 lines above. To generalise to N lines above:

POSIXLY_CORRECT=1 awk -v N=2 -v X=2 '
  BEGIN{N--}
  NR > N {
    if (/^output-[0-9]{3}\.ts$/ && ++n % X == 1)
      print "#Inserted line", i++
    print l[NR % N]
  }
  {l[NR % N] = $0}
  END{
    for (i = NR > N ? NR - N + 1 : 1; i <= NR; i++) print l[i % N]
  }'

Answer 2

Perl to the rescue:

< .m3u8  perl -ne 'push @w, $_;
                   unshift @w, "#Inserted line " . (0 + $c++) . "\n"
                       if /^output-[0-9]+\.ts/ && 0 == $m++ % 2;
                   print shift @w while @w > 1;
                   }{ print @w;'

@w is the "sliding window" array that keeps the last 2 lines. $m contains the number of matches, i.e. the number of "output-XXXX" lines encountered so far, so if modulo 2 it gives 0, you pick up every 2nd match. unshift inserts a line to the window array at the beginning, shift retrieves it. At the end, you have to output the remaining lines.