# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(me)(?!([^<]+)?>)/<span class='highlight'>$1<\/span>/gi;t' <<< "I have ever been prone to seek adventure and to investigate and experiment where <span class='me'>wiser men</span> would have left well enough alone."
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html