$re = '/\$plugins\.(\w+)\.(.+)/';
$str = '$plugins.official.random_hsv_color_as_rgb.RandomHsvColorAsRgb';
$subst = "plugins.\1.process_conditions.variabilities.\2";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$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