schema pattern matching (negate)

Hi,

I am trying to answer a post from the comp.text.xml newsgroup.

question asked how to negate strings in XSD pattern matching regular
expressions

I answerd to use caret ^

e.g.
[^S]

but what is required is that pattern does not begin with a whole string e.g.
"STR1234" and "STRxccc" are bad (because of STR) but "SAT1234" and "CARxccc"
are good (not beginning with STR).

I tried ^(STR) but this does not seem to work and

[^S][^T][^R]

does not allowe.g. "SAT"

Any ideas please?
Colin