In Texis version 7.07.1584374000 20200316 and later, the escape \<nomatch\> may be given as the sole contents of a REX
expression. This will match and return non-empty data that is not
returned by any other (non-\<nomatch\>) expression.
Since it is a negation, it may only be given if other (non-\<nomatch\>) expressions are given as well, e.g. with
<rex> in Vortex. This may be useful when parsing text with
multiple complex expressions, as a catch-all to match remaining text/space
etc. that "falls through".