VBA CODE to show leading zeros

We require a VBA code to:

Add a leading zero to a fixed length string amongst variable length strings.

E.G. 123456789 to become 0123456789 - it is to look for 9 digit numbers and add the leading zero to those only.

if the code encounters 123456 it is to remain unchanged and if it encounters ABC12345 again it is to remain unchanged. If there are shorter numbers and if there are shorter letters and numbers it is to ignore those.