$re = '/\"((http|https)\:\/\/([^\"]+)\.key)\"/m';
$str = '<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>
';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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 PHP, please visit: http://php.net/manual/en/ref.pcre.php