$s = 'aaaabaaaabaaaaabaaaab';;
$s =~ m[(.+).*\1] and print $1;;
aaaabaaaab

##</code><code>##

aaaabaaaabaaaaabaaaab
aaaabaaaab aaaabaaaab
1         ^2

##</code><code>##

$s = 'aaaabaaaabaaaaabaaaab';;
$s =~ m[(.+)\1] and print $1;;
aaaab

##</code><code>##

aaaabaaaabaaaaabaaaab
aaaabaaaab aaaabaaaab
1    2    ^3    4