\p{Numeric_Value=-1/2}

[Karl Williamson]

U+0F33 TIBETAN DIGIT HALF ZERO has a numeric value of -0.5.  (I believe 
the existence of this character in the wild is apocryphal however.) 
There is no rule against other code points becoming encoded with a 
negative value.

However, UTS 18 says the hyphen-minus sign is supposed to be ignored 
within \p{} constructs, leaving no way to legally specify negative values.

I suspect that UTS 18 should be clarified to indicate that the hyphen 
minus at the beginning of a number should not be ignored, even with 
loose matching.  But then what to do about two in a row?