This might be somewhat buggy behavior, but here is how I am interpreting the events. Because of your {2}, the pattern you are ultimately trying to match is /((\w+))((\w+))/. However, as YAPE::Regex::Explain points out,
NOTE: because you are using a quantifier on this capture, only the LAST repetition of the captured pattern will be stored in [$2]
On the first time through, \w+ grabs both letters are stores them in $2, and prints ab as expected. It then sees the repetition at the end, shifts the pointer for the second buffer to the second repetition (what would be $4 in my unrolled version). This means when the first attempt fails and you grab a on your second attempt, $2 doesn't point there anymore - it points to the second buffer in the second iteration.

I think (though this is subject to argument) that the correct behavior should not be what you claim, but should be 

$2 not defined
$2 not defined
$2=b

[download]
since the final $2 buffer is not populated until your second iteration. In the end, it just goes to emphasize perlreftut's warning:
Be warned that this feature is considered experimental, and may be changed without notice.

In reply to Re: A bug in Perl regex(?) by kennethk
in thread A bug in Perl regex(?) by Serge314

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