Regular Expression - Back reference

I would like to search for digits in a tagfield. If the digits are followed by characters or the digits are behind characters, I want to make a white space between the digits and the characters.

In RegxBuddy this works:

Match: (\d)(?=[a-zA-Z])|([a-zA-Z])(?=\d)
Replace: \0

But If I try to enter this on Mp3Tag, the result is really strange... :unsure:
How do I make a back reference in Mp3Tag?

As written in the help $N is used to get the reference:
Replace: $0

Works great! Thanks for this important information. :w00t:

Is this $0 something special for Mp3Tag?
According to my Regex-documentation :book: , the $n is a "reference capture" and the \0 a "numbered backreference" - not exactly the same.

Which regex-dialect is used by Mp3Tag?

\N (e.g. \1) can be used to reference the Nth sub-expression.

It's Perl syntax.