Hello and welcome,
It seems that the problem here is padding with a regular expression.
Using two of them should do the trick. For example :
%b(s/-(\d)-/-0\1-/g)(s/([0-9]+)-([0-9]+)-([0-9]+)/\3-\2-\1 \4/).%e
the first regular expression "s/-(\d)-/-0\1-/g" will pad one digit substrings with a "0" and give its result to the second one (yours).
But you can achieve this without regular expressions. For example with this one :
%N3-%N2{2}-%N1{2} - %b[-1].%e
%N3 : extracts the 3rd number of the base name
%N2{2} : extracts the 2nd one and pad it to two digits
%N1{2} : extracts the 1st one and pad it to two digits
%b[-1] : extract the last element from the base name when considering that elements are separated by "-"
Hope this will help,
Rémi