> reading the last capture with embedded Perl code like /(....(?{print $1})){4}/

demo: (using /x for clarification)

  DB<66> $x = q[AAD34017837201D98AAED18778DEF993];

  DB<67> $x =~ m/(?: (....) (?{say $1}) ) {4} /x
AAD3
4017
8372
01D9

  DB<68>

[download]

update

I think this explains your "mystery", they all match but only the last one is kept in $1.

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

In reply to Re^2: mysteries of regex substring matching by LanX
in thread mysteries of regex substring matching by smile4me

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