<dfdl:assert testKind="pattern">ABC</dfdl:assert>

won't compile because ABC doesn't match the syntax of an "expression" in our expression language.

However, in this case it's supposed to accept a pattern there.

DFDL-7-040R : DFDL asserts can be placed on
DFDL-7-041R : These dfdl:asserts contain a t
DFDL-7-042R : The dfdl:assert is said to be
DFDL-7-043R : An unsuccessful dfdl:assert ca
DFDL-7-044R : The dfdl:testKind attribute sp
DFDL-7-045R : The expression or pattern can
DFDL-7-046R : 7-046R - It is a schema defini
DFDL-7-047R : 7-047R - It is a schema defini
DFDL-7-048R : A dfdl:assert can be an annota
DFDL-7-049R : •    an xs:element reference,
DFDL-7-050R : •    an xs:group reference,
DFDL-7-051R : •    an xs:sequence
DFDL-7-052R : •    an xs:choice.
DFDL-7-053R : More than one dfdl:assert may