# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/\"((http|https)\:\\/\\/([^\"]+)\.key)\"//gm;t' <<< "<a href=\"http://site.com/palavra.key\"></a>
<a href=\"https://site.com/palavra.key\"></a>
<a href=\"http://subdominio.site.com/palavra.key\"></a>
<a href=\"https://subdominio.site.com/palavra.chave\"></a>
<a href=\"http://subdominio.site.com/uma_coisa_qualquer.keykey\"></a>
<a href=\"https://subdominio.site.com/outra_coisa_qualquer.key\"></a>
<a href=\"https://subdominio.site.com/outra_coisa_qualquer.keys\"></a>
"
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