s///.0     ->    s/// . 0
s///..0    ->    s/// .. 0
s///...0   ->    s/// ... 0
s///...0   ->    s/// ... .0   ->    s/// ... 0

##</code><code>##

>type file
123
abc
def

>perl -ne"print if s/.*?(?=abc)//.0" file
abc

>perl -ne"print if s/.*?(?=abc)//..0" file
abc
def