If $test isn't part of a larger string, you can just slice it instead of using a regular expression.

$test="LDAP://CN=MyPc,OU=MyOu,DC=NyDomain,DC=com";
$CN = substr($test, 10, index($test, ",") - 10);
print "$CN\n";
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You can slice off the "LDAP://CN=" since you know it'll always be there, then you slice to the first comma.