back references in syntax region skip/end patterns

Hi all,

I'm trying write a syntax file for (a subset of) YAML, mainly to
provide nice folding. It would not be so hard (I think) if I could use
back references in the skip and end patterns (referring back to the
start pattern), but that appears not to be allowed. Does anybody know
a way to do this? For example, I want to fold:

key 1:
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do
eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad
minim veniam,
...
key 2:
Nam liber tempor cum soluta nobis eleifend option congue nihil
imperdiet
doming id quod mazim placerat facer possim assum. Typi non
habent claritatem
...

... like this:

+-- 4 lines: key 1
+-- 4 lines: key 2

Cheers, Roald

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