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

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(.) infront of numbers with a space.(e.g. 12.hello.mp3 => 12 hello.mp3). If I have regex as "[0-9].", it replaces number also. Please help me.

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)

I tested with vscode. You must use groups with parentheses (group of regex)

Practical example

• start with sample data

1 a text
2 another text
3 yet more text

• Do the Regex to find/Search the numerical digits and spaces. The group here will be the digits as it is surrounded in parenthesis

(\d)\s

• Run a replace regex ops. Replace spaces for a dash but keep the numbers or digits in each line

$1-

• Outputs

1-a text
2-another text
3-yet more text

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