Interesting discussion! ICU does not support "is" nor "in" prefixes. I wasn't even aware that UAX #44 loose matching prescribes "is". ICU just implements what Property[Value]Aliases.txt say:
# Loose matching should be applied to all property names and property values, with # the exception of String Property values. With loose matching of property names and # values, the case distinctions, whitespace, hyphens, and '_' are ignored. The prefixes seem gratuitous and confusing. For example, if I read UAX44-LM3 right, it would allow [:isscript=isgreek:]. We do support just [:Greek:] for scripts and [:L:] for general categories. I would rather not add support for the prefixes in ICU. markus
