Well, usually I wouldn't mention it but since it comes pre installed last I looked, you might always try Perl instead of having to install the GNU Stream EDitor.
Code:
Terry@dixie$ echo 'gratefull' | perl -pe 's/gratefull/grate\x2dfull/' 4:54
grate-full
Terry@dixie$ 4:55
Which I assume is what your trying to do but upon document.txt instead of stdin with sed.
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Thou shalt check the array bounds of all strings (indeed, all arrays), for surely where thou typest ``foo'' someone someday shall type ``supercalifragilisticexpialidocious''.