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

aaaabaaaabaaaaabaaaab
aaaabaaaab aaaabaaaab
1         ^2

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

aaaabaaaabaaaaabaaaab
aaaabaaaab aaaabaaaab
1    2    ^3    4