How to get the last element of a string with : as seperator using Xpath1.0 in a XML doc

I have this xml document: " : " is the separator.

xml = """
<a>
    <b> 1,2,3 : 4,5,6 : 7,8,9 : 10,11,12 </b>
</a>
"""

How can I get the middle number in the last pair (i.e., 11) by using Xpath 1.0

I used the combination of substring-before and substring-after, but the result didn't work out as I wanted.

Update: Sometimes, it's not always only 4 pairs, sometimes only 1 pair, or 2 pairs, but maximum it can be up to 10 pairs. So all I want is to always get the middle number of the last pair out. How can I extract it?

Thank you for any ideas.

In 1.0 you would need a larger xpath like this:

substring-before(
    substring-after(
        substring-after(
            substring-after(
                substring-after(/a/b,':')
                ,':')
            ,':')
        ,',')
    ,',')