Wow, that's what can be called rapid development :-), thanks very much!
I did'n noticed before, that \G had been implemented already.
\X works fine for me, it also maintains the input string indices correctly.

We can use unicode character properties \p{Letter} and unicode bloks \p{inBasicLatin} properties; 
the script properties like \p{Latin} or \p{IsLatin} return "undefined property name".
I guess, this would require the access to the respective information in unicodedata, where it isn't available now (there also seem to be much more scripts than those mentioned at regular-expressions.info
cf.
http://www.unicode.org/Public/UNIDATA/Scripts.txt
http://www.unicode.org/Public/UNIDATA/PropertyValueAliases.txt (under "# Script (sc)").

vbr