Find text between lines in linux

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http://serverfault.com – I have a log-file where at the end of a series of lines you can see if this block is relevant now I'm looking for a command like sed to delete de blocks ending with "Content-Length: 0" and beginning with the last "--" before this line. I tried sed -n "/--/,/Content-Length: 0/d" but this takes the first "--" and the first "Content-Length: 0" and deletes it. ex : line 1 "--" line 2 line 3 "Content-Length: 20" line 4 "--" line 5 line 6 "Content-Length: 0" i want to delete line 4,5 and 6 not line 1 to 6 hohw can i do this? (HowTos)