$text = "asssssaasssxaasaayaaa";
print "Before: $text\n";
$text =~ s/(?:\G([^xa]*?)a|()a(?!.*y))/$1A/g;
print "After: $text\n";
print "-----\n";
$text = "asssssaasssxaasaxayayaaa";
print "Before: $text\n";
$text =~ s/(?:\G([^xa]*?)a|()a(?!.*y))/$1A/g;
print "After: $text\n";
print "-----\n";
$text = "asssssaasssxaasayaxayaaa";
print "Before: $text\n";
$text =~ s/(?:\G([^xa]*?)a|()a(?!.*y))/$1A/g;
print "After: $text\n";