Just to note: since $1 and its ilk will keep their values if the regex fails to match, make sure you wrap this all in a conditional. I'm not sure about what's so bad about using length here; the grouping number shift is going to be a problem anyway, if your code's not setting $regexp, how will you know whether it has groupings at all? (and if so, how many). Of course, if you want to nab those matches, you can always use a nice listy assignment like:

if (my ($foo, $bar, @reg_groups) = $string =~ /^(.*?)($regexp)/) {
  # $foo = $1, $bar = $2, @reg_groups = all the matching groups from $
+regexp
 my $totength = length $foo.$bar;
 my $foolength = length $foo;

    # whatever happens next
}
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HTH