$re = '~<a\s[^>]*href="https?://[^>]*>(.*?)</a>~is';
$str = '<a href="https://site.ru/*****>слово</a>
<a class="m" href="https://site.ru/*****>слово</a>
<a href="https://site.ru/*****" class="m" >слово</a>
<a href="https://www.site.ru/sistemy-promyshlennogo-konditsionirovaniya/fankoyly/ballu-machine/" rel="nofollow">слово</a> <br>
<a href="https://www.site2.ru/fdescript/1559/?design=1">слово</a> <br>
<a href="https://www.site3.ru/fdescript/997/?design=1">слово</a> <br>
<a href="https://www.site3.ru/fdescript/1003/?design=1" rel="nofollow">слово</a>
';
$subst = "\1";
$result = preg_replace($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