Are you trying to determine whether a regex had capture groups or not?

Thanks - that's very interesting (@LAST_MATCH_START).

Through some code I am currently working on I discovered a potential problem here, there is a subtle but very important distinction between @- and @+, from the former:

One can use $#- to find the last matched subgroup in the last successful match. Contrast with $#+, the number of subgroups in the [last successful] regular expression.

$ perl -MData::Dump -e '"ya"=~/(x)?/; dd $#-, \@-, $#+, \@+'
(0, [0], 1, [0, undef])
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And updating my previous example:

"1" =~ /1/    or die;
print $#-?"yes\n":"no\n";  # -> no
"1" =~ /(1)?/ or die;
print $#-?"yes\n":"no\n";  # -> yes
"1" =~ /(2)?/ or die;
print $#-?"yes\n":"no\n";  # -> no !!
"1" =~ /(2)?/ or die;
print $#+?"yes\n":"no\n";  # -> yes
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