[Freeswitch-users] empty var expression

I tried this for an var marked as "UNDEF" in log <condition field="${var}" expression="^UNDEF$" break="never"> <condition field="${var}" expression="UNDEF" break="never"> <condition field="${var}" expression="undef" break="never"> <condition field="${var}" expression="^undef$" break="never"> <condition field="${var}" expression="" break="never"> <condition field="${var}" expression="^$" break="never"> <condition field="${var}" expression="[UNDEF]" break="never"> <condition field="${var}" expression="[]" break="never"> without success. which is the right expression for empty var please ? Thanks