better exception when boost regex cannot initialize ICU support - closes mapnik#2722

Author: Dane Springmeyer

src/expression.cpp

@@ -40,18 +40,29 @@ expression_ptr parse_expression(std::string const& str)
     auto node = std::make_shared<expr_node>();
     std::string::const_iterator itr = str.begin();
     std::string::const_iterator end = str.end();
-    try {
-        bool r = boost::spirit::qi::phrase_parse(itr, end, g, space, *node);
-        if (r && itr == end)
+    bool r = false;
+    try
+    {
+        r = boost::spirit::qi::phrase_parse(itr, end, g, space, *node);
+    }
+    catch (std::exception const& ex)
+    {
+        if (std::string("boost::spirit::qi::expectation_failure") == std::string(ex.what()))
         {
-            return node;
+            // no need to show "boost::spirit::qi::expectation_failure" which is a std::runtime_error
+            throw config_error("Failed to parse expression: \"" + str + "\"");
         }
         else
         {
-            throw config_error("Failed to parse expression: \"" + str + "\"");
+            // show "Could not initialize ICU resources" from boost::regex which is a std::runtime_error
+            throw config_error(std::string(ex.what()) + " for expression: \"" + str + "\"");
         }
     }
-    catch (std::exception const&) // boost::spirit::qi::expectation_failure
+    if (r && itr == end)
+    {
+        return node;
+    }
+    else
     {
         throw config_error("Failed to parse expression: \"" + str + "\"");
     }