c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 for my $s (qw(aaxyxyxy aaaxyxyxy aaaaxyxyxy)) {
   my $match = $s =~ m{
     \b (a+) ((?: xy)+) \b
     (?(?{ 2 * length($1) != length($2) }) (*FAIL))
     }xms;
   print $match ? '   ' : 'NO ', qq{match '$s'};
   }
"
NO match 'aaxyxyxy'
   match 'aaaxyxyxy'
NO match 'aaaaxyxyxy'

##</code><code>##

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 for my $s (qw(aaxyxyxy aaaxyxyxy aaaaxyxyxy)) {
   my $match = $s =~ m{
     \b (a+) (?{ length $^N }) ((?: xy)+) \b
     (?(?{ 2 * $^R != length $^N }) (*FAIL))
     }xms;
   print $match ? '   ' : 'NO ', qq{match '$s'};
   }
"
NO match 'aaxyxyxy'
   match 'aaaxyxyxy'
NO match 'aaaaxyxyxy'