Conditional Rename

Hello,

I have a folder with a lot of files. Mainly they are something like that:

XX_11_123_123456_STR.txt
and
22_456_123456_STR.txt

How can I rename files without initial XX_ with YY_ ? Basically
22_456_123456_STR.txt should become YY_22_456_123456_STR.txt and the other files remain the same.

Thanks!
Alex

Replace: ^(?!XX_)
with: YY_
Case sensitive
Use regular expressions