rename second word in filename

Hi,

I have used regular expressions in a far far past but can't seem to work out my issue by myself.
In the process of renaming all my phto and videoclip filenames I want them to have the following format:

20140704_16.25.58.mp4

while they presently are like
20140704 162558.mp4

It must be very simply but to this moment my results are poor.

any help is highly appreciated

Replace: ([^ ]*) (\d{2})(\d{2})(\d{2})
with: \1_\2.\3.\4
Use regular expressions