Further to tybalt89's comment:

... your regex works ... works great for unsigned numbers ...
[…]
... it does not work with unsigned numbers ...

I assume you mean signed numbers in the second case; if so, please supply representative test cases: just what output do you expect for -255, -256, -65535, -65536, etc.

And I would strongly suggest concentrating on edge cases in your test cases, e.g. something like
    0, 1, 255, 256, 65535, 65536, 16777215, 16777216, 4294967295
rather than
    2 20 200 2000 20000 200000

Update: Also, are you aware that the sprintf 'x' format conversion is essentially unsigned: 

Win8 Strawberry 5.8.9.5 (32)  Mon 08/30/2021 18:24:36
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings
printf "%08x \n", -256;
printf "%08x \n", 4294967040;
^Z
ffffff00
ffffff00

[download]

Give a man a fish:  <%-{-{-{-<

In reply to Re^7: Split any number into string of 8-bit hex values (=1 byte) (updated) by AnomalousMonk
in thread Split any number into string of 8-bit hex values (=1 byte) by drsweety

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