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$\ = "\n";

...
print '!aabcc' =~ /^$re/ ||0;  # 0  First call is anchored to start.
print 'a!abcc' =~ /^$re/ ||0;  # 0  Recursive call is anchored to pos.
print 'aa!bcc' =~ /^$re/ ||0;  # 0  Recursive call is anchored to pos.

$, = ", ";
$\ = "\n";
print 'aab' =~ /\Ga/g;  # 'a', 'a'.
print 'aba' =~ /\Ga/g;  # 'a'.      Anchored at pos.
print 'baa' =~ /\Ga/g;  # Nothing.  Also anchored to start.