Accented characters are not easy to manage since they are encoded in UTF-8 and they are 2-bytes in size: using octal or hexadecimal codes to match them may become a mess. On the other hand, if you can type them in the command line, you can always translate them literally, e.g.
echo Renée | sed 's/é/e/'
but if you want to do this for every accented character, umlaut, caron, etc. etc. in text files, you need to write down a sequence of long character lists, e.g.
sed -e 's/[èéêë]/e/' -e 's/[àáâãäå]/a/' ...
A more convenient way is by means of the iconv
command to change the character encoding from UTF-8 to ASCII and transliterate the special characters so that é become e, ò becomes o and so on. Example:
echo Renée | iconv -f UTF-8 -t ASCII//TRANSLIT
or if you want to change the content of a file:
iconv -f UTF-8 -t ASCII//TRANSLIT infile > oufile
Hope this helps.