$re = '/\.\s/';
$str = '01. Title (feat. John Doe).mp3
4. Song (feat. Jane.Doe).flac
23. Name.Title.m4a';
$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