while (/(b*)/g) {
...
}
##</code><code>##

$perl -e '$_="abbbabc";
while (/(b*)/g) { print "($1)\n" }'
()
(bbb)
()
(b)
()
()

##</code><code>##

abba
| match zero b's before a
^abba
 | match zero b's again. Don't substitute here, bump along
^abba
  | match 'bb'
^a^a
   | match zero b's
^a^^a
    | match zero b's, don't substitute but bump along
^a^^a^
     | match zero b's, don't substitute but bump along

##</code><code>##

$ perl -le '$_ = "abba"; s/b*/print pos; "^"/eg; print'
0
1
3
4
^a^^a^