While replacing using regex, How to keep a part of matched string?

Question

I have

12.hello.mp3
21.true.mp3
35.good.mp3
.
.
.

so on as file names in listed in a text file.

I need to replace only those dots(.

Answer 1

Using the basic pattern, well described in the accepted answer here is an example to add the class="odd" and class="even" to every <tr> element in Notepad++ or any other regex compatible editor:

Find what: (<tr><td>)(.*?\r\n)(<tr><td>)(.*?\r\n)

Replace with: <tr class="odd"><td>\2<tr class="even"><td>\4

Answer 2

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)