On Sun, 09 Mar 2008 16:19:57 -0400, Ron Rosenfeld
wrote:

=IF(AND(ISNUMBER(-LEFT(A1,11)),ISTEXT(RIGHT(A1,1)),LEN(A1)=12),
TEXT(LEFT(A1,11),"00-000000000-")& RIGHT(A1,1),"invalid entry")

Better with:

=IF(AND(ISNUMBER(-LEFT(A1,11)),ISERR(-RIGHT(A1,1)),LEN(A1)=12),
TEXT(LEFT(A1,11),"00-000000000-")& RIGHT(A1,1),"invalid entry")

--ron