I found all the solutions interesting (even those that let 'ab' slide
through). For my own entry, I wanted to do it all in one regex and
succeeded but the initial cost was nearly 90 characters. After a
dinner and beer break I managed to simplify it out (I was using an
extra grouping that wasn't needed), and was then able to use a bit of
japhy's techniques to squeeze down to tie at 74:
sub is_fibo {
pop=~/^(?{$x=$y=0})(?:(.)(??{"\Q$1"x$y})(?!\1)(?{$y=($x+=++$y)-$y})){3
+,}$/
}
Even passes 'strict' in 5.6.1, but not in 5.00503. Of course, there
is the distinct possibility that my brain is mushier than I think
from the beer and I'm missing some obvious failure mode with this
one.
Update: Doh! And along comes japhy to casually make a putt I
completely missed for a 2 stroke savings ... probably only used one
hand too :-)