$- is the offset of the start of the last successful match.
You have my $re1 = qr/((a+)?(b+)?(c))*/;
Your outer capture group may be repeated zero or more times. In the case of your test string, "aacbbbcac", it matches three times:
At the first repeat, it matches "aac" with $1 being "aac", $2 being "aa", $3 being undef (not matched) and $4 being "c", but because of the repeat count it doesn't stop there, so you never see these values.
At the second repeat $1 is "bbbc", $2 remains "aa" (group (a+) didn't match in this repeat but $2 is the 'last successful match'), $3 is "bbb" and $4 is "c", but it doesn't stop there, so you don't see these values either.
The third and final repeat sets $1 to "ac", $2 to "a" leaves $3 as it was at the last successful match (i.e. "bbb") and sets $4 to "c".
So, the issue is that the capture groups return the last successful match rather than the last match or failure as the case may be.
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Re^4: Explain a regexp matched group result
by jdd (Acolyte) on Oct 28, 2013 at 19:24 UTC