$re = '/(?<!\d|^)(?<!(?<![^\W_])(?:sha|md))(?=\d)|(?<=\d)(?!\d|$)|_/m';
$str = 'Aisha256
ai_sha256
test123
84test
test md5
sha256
word two sha1
w0rd
test_md5
sha256
md5';
$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