Read as: "lookahead (and capture) 3 digits; then step forward 1 anything". 

$s = '1234567890';;

print $1 while $s =~ m[(?=(\d{3})).]g;;
123
234
345
456
567
678
789
890

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In reply to Re: Lookahead assertion by BrowserUk
in thread Lookahead assertion by talexb

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