What you're saying with "/(b)*?/" is that you want to match "b" and store it in $1, and do this zero or more times, but no more times than necessary. As ikegami notes, what you may mean is "/(b*?)/, which says that you want to match "b" zero or more times, but no more than necessary, and store the collection of characters matched in $1.

The difference is that the first might be trying to store any number of different single instances of "b" in $1, all of which are matched, it may even store no instances of "b" in $1. It may have trouble deciding which single instance to put in $1. It looks like back in 5.6.1 the regular expression engine gets really confused by this, as well it might.

In 5.8.x versions it looks like it decides to take the first instance, which is "no" occurances of "b".

Adendum for those who know the RE engine: I know this is not how the RE engine works on the string, it's meant as an explanation of why the expression is ambiguous, rather than how the RE engine works (which the general reader of an RE shouldn't have to know, IMO). The 5.6.1 engine has a bug here, but it is a lot to ask of the little engine that could.