[mzmm]

you could try this:

find:
(?<=\P{Greek})(\p{Greek}+)-(?!\1)

replace:
\1

it's looking one or more greek characters in a capturing group:

(\p{Greek}+)

that are preceded by anything other than a greek character:

(?<=\P{Greek})

then a hyphen:

-

that is not followed by the group it matched previously:

(?!\1)

replacing it with \1 just removes the hyphen

** edit **

i was trying to get this to work with unicode ranges so that it could be simplified further (no need for the look-behind), but couldn't seem to get it working in sigil, or my other text editor which has PCRE, for that matter.

i was trying to match [\u0370-\u03FF] and (?-u)[\u0370-\u03FF] with no success. anyone have tips on this?

** edit 2 **

i was hoping to get rid of the look-behind by starting the expression with a word boundary, but turns out \b is only useful for ASCII characters, i.e. [a-zA-Z0-9_], so looks like the look-behind may be necessary in these cases.

here's an updated version based on Doitsu's comment below that includes Greek_Extended in the search pattern:

(?<![\x{0370}-\x{03FF}\x{1F00}-\x{1FFF}])([\x{0370}-\x{03FF}\x{1F00}-\x{1FFF}]+)-(?!\1)