how to match lines without some strings in sed

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http://stackoverflow.com – I tried to print out the first line (started with > as the files are fasta files) of every file under the folder and use sed to extract out those lines without plasmid and fragment. I did ls * | xargs -I one sed -n -e '/^>.*[^fragment|plasmid].*/p' one > outfile but it dosen't work? How should I modify it? (HowTos)