I'm pretty sure that is checking for Greek 'letters' not words. As long as you are using the Greek alphabet and not a romanization/translation it should work fine.
Code:
\p{ name } Matches any single character in the Unicode general category or named block specified by name.
ex:
\p{Lu} "C", "L" in "City Lights"
\p{IsCyrillic} "Д", "Ж" in "ДЖem"
\p{IsGreekandCoptic}
That is a .NET regex reference...and I can't for the life of me remember which regex flavor Sigil uses...
edit:
Here is a link to a unicode regex reference list for anyone else looking.