$\ = "\n";

my $re;
$re = qr/ a (??{ $re }) c | b /x;

print 'aabcc'  =~ /^$re/ ||0;  # 1
print '!aabcc' =~ /^$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.