You have to use a negative lookbehind (see
this site, for example)
Code:
(?<!">\s—\s«\s.+?)(\.\s«\s)
Your selection is in the group 1 (i.e.\1), if you want to make a substitution
Explanation :
(?<! expr) ==> negative lookbehind (fail if "expr" is found)
">\s—\s«\s.+? ==> if found, don't select the expression that follows this group)
in witch .+? means a group of any char (at least one char) until the next expression (not greedy)
(\.\s«\s) ==> the expression you're looking for (it is the group 1)
Edit : In the search options, "Dot all" must be unchecked