Make regex const.

src/search.cpp

@@ -60,7 +60,7 @@ std::vector<match> search(const fs::path &filepath, std::string_view regex,
         flags |= boost::regex::icase;
     }
 
-    boost::regex re(regex.data(), flags);
+    const boost::regex re(regex.data(), flags);
     std::vector<match> matches;
     for (const auto &entry : zip::list(filepath))
     {