Remove trailing and leading numbers

1

i need to remove all trailing and leading numbers from a string but only from where numbers are not associated with any string.

For EX:
input : 6,a.RPP9100,a.RPP0040,a.RPP4160,a.RPP6180,a.RPP8190,a.RPP9100,100
output : a.RPP9100,a.RPP0040,a.RPP4160,a.RPP6180,a.RPP8190,a.RPP9100

input : a.RPP9100,a.RPP0040,a.RPP4160,a.RPP6180,a.RPP8190,a.RPP9100,100*8,9
output : a.RPP9100,a.RPP0040,a.RPP4160,a.RPP6180,a.RPP8190,a.RPP9100

Kinldy suggest.