"aaaa" =~ /a*a*b/

##</code><code>##

"aaaa" =~ m/ (a*) (a*) (?{ print "Tried $`($1)($2)\n"; }) (?!) /x;

##</code><code>##

Tried (aaaa)()
Tried (aaa)(a)
Tried (aaa)()
...
...
Tried aaa()(a)
Tried aaa()()
Tried aaaa()()

##</code><code>##

24670925422945900903156716 (= 2e25)