This feature has been discussed frequently as something missing from DFDL.
There are formats where this behavior would be very convenient.
It has been suggested that this feature is needed for JPEG2000 format.
This ticket created to track the issue. However, I'm not convinced that dfdl:lengthKind 'pattern' isn't a universal work around for the lack of this feature. If so then the priority of this becomes very low.
It may be that this feature is required because of the lack of streaming-behavior (unlimited data size) for regex matching in dfdl:lengthKind 'pattern'.
It is possible our regex matching needs to be rewritten in terms of the Scanner class (which operates on InputStream not CharSequence. Existing code uses CharSequence, which due to nature of the interface, must be a finite object, not a stream that is potentially unbounded.