editor: don't allow bad location id strings

in particular using spaces or commas here results in invalid map files when the map is written to disk. (repoted on the forum)

src/editor/palette/location_palette.cpp

@@ -21,11 +21,15 @@
 #include "font/standard_colors.hpp"
 #include "tooltips.hpp"
 
+#include "editor/editor_common.hpp"
 #include "editor/toolkit/editor_toolkit.hpp"
 #include "gui/dialogs/edit_text.hpp"
+#include "gui/dialogs/transient_message.hpp"
 
 #include "formula/string_utils.hpp"
 
+#include <boost/regex.hpp>
+
 static bool is_positive_integer(const std::string& str) {
 	return str != "0" && std::find_if(str.begin(), str.end(), [](char c) { return !std::isdigit(c); }) == str.end();
 }
@@ -259,7 +263,18 @@ void location_palette::adjust_size(const SDL_Rect& target)
 		button_add_.reset(new location_palette_button(video(), SDL_Rect{ target.x , bottom -= button_y, target.w - 10, button_height }, _("Add"), [this]() {
 			std::string newid;
 			if (gui2::dialogs::edit_text::execute(_("New Location Identifier"), "", newid)) {
-				add_item(newid);
+				static const boost::regex valid_id("[a-zA-Z_]+");
+				if(boost::regex_match(newid, valid_id)) {
+					add_item(newid);
+				}
+				else {
+					gui2::show_transient_message(
+						_("Error"),
+						_("Invalid location id")
+					);
+					//TODO: a user visible messae would be nice.
+					ERR_ED  << "entered invalid location id\n";
+				}
 			}
 		}));
 		button_delete_.reset(new location_palette_button(video(), SDL_Rect{ target.x , bottom -= button_y, target.w - 10, button_height }, _("Delete"), nullptr));