# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/<!--(.|\s)*?-->//gm;t' <<< "blablabla
<teste>teste</teste>
etc
<!-- Assinatura principal -->
<x>mais conteúdo</x>
<!-- [if gte mso 9]><xml> <o:OfficeDocumentSettings> <o:AllowPNG /> </o:OfficeDocumentSettings> </xml><![endif]-->
blablabla
<!---->
<!--
-->
<p>lorem ipsum</p>
<!-- <!-- fad ->----->"
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