Filename help

Newbie here, a litle confused too....

Basically, I am trying to bulk rename a load of mp3's
The current name format is "YYYY.MMDD Title Text" - where Y= year M= Month & you guessed it, D= Day

I would like to rename the files with "-" between the Year, month and Day, leaving title text there too. For example "YYYY-MM-DD Title Text"

Could anyone help me with the action scripting please?

Many thanks

Action: Format value
Formatstring: $regexp(%_filename%,'^(\d\d\d\d).(\d\d)(\d\d)\s(.+)$','$1-$2-$3 $4')

2011.1027 Title Text
2011-10-27 Title Text