sed(1) / BRE bug?

[tlaronde%polynum.com@localhost]

Hello,

I have been surprised by the following (I made what I consider a mistake
in a RE with sed(1) but it went unnoticed under NetBSD since it
succeeded when, I think, it should not):

$ echo '$(VAR)' | sed 's/^$(\([^)][^)]*\))$/\1/'
VAR

The problem? the leading '$' is not escaped (I was trying to get the var
name from a Makefile)...

Is this a bug or is this behavior undefined or even required by
POSIX?

Best,
-- 
        Thierry Laronde <tlaronde +AT+ polynum +dot+ com>
                     http://www.kergis.com/
                       http://www.sbfa.fr/
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