rename - change n'th character of regexp

Hi,

I wonder if its possible to do the following task using rename (perl v5.8.8).

I want to find filenames matching the specific pattern and then change chosen character of this pattern to a given character, e.g. do the following renaming:
regexp: 'a[12]b' -----> 'a0b'

What's the simplest solution for this? Is one-line rename possible?

Thx!