Looks like you have three groups of symbols - digits , lower and uppercase characters - and every position stays within its group.

If a new position is needed it'll adjust to the leftmost group.

This makes sense to me, because the structure of a log_89ABCacd.txt file will always look the same, with only the numbers growing.

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

In reply to Re^3: How the auto-increment operator works? by LanX
in thread How the auto-increment operator works? by zapdos

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