$re = '/\#\#\ (\d(\.\d)?(\.\d)?)/m';
$str = '# Changelog
## 1.2.3';
$subst = "## [$1](StorybookURL/$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