# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/<div class="whatever">(.*?)(<div[^>]*>((?>(?:(?!<\\/?div)[^\n])+|\n|(?2))*)<\\/div>)(.*?)<\\/div>/\1\2\4/gmus;t' <<< "
<div class=\"tutu\">
Blahblah.
<div class=\"whatever\">
Blahblah0.
<div class=\"tata\">
Blahblah1.
</div>
Blahblah2.
</div>
Blahblah3.
</div>
"
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