Replace
^(\d+)\.(.*mp3)$
with
\1 \2
Also, in recent versions of notepad++, it will also accept the following, which is also accepted by other IDEs/editors (eg. JetBrains products like Intellij IDEA):
$1 $2
This assumes that the notepad++ regex matching engine supports groups. What the regex basically means is: match the digits in front of the first dot as group 1 and everything after it as group 2 (but only if it ends with mp3)