language: Perl (perl 5.16.2)
date: 332 days 0 hours ago
link:
visibility: public 
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$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";