How To Add A Dot After Track Number

I was wondering how you would do this,

15goodsong -> 15. Good Song or 15. goodsong

the goal is to get the . after the # so if I have,

4 The Best Song Ever -> 4. The Best Song Ever

This is for the Filename field (edit)

Thanks!

Create an action of the type "Replace with regular expression" for the field.
Search string: ^(\d+)
Replace string: $1._
(where the _ is a blank - of )
If we are talking about the filename, you should use the function Convert>Tag-Filename

Also, it could be necessary to remove double-blanks afterwards.

What do you mean by "tracktitle", ...
does it be the tagfield TITLE, ...
or does it be the pseudo tagfield _FILENAME?
Why don't you create the target string fresh from the TRACK number and TITLE string?

TITLE <== $regexp(%TITLE%,'^(\d*)\s*(.*)$','$1. $2')

DD.20170311.1950.CET

This worked, thanks I love you!