fixed re2c warnings

Makefile

@@ -67,7 +67,7 @@ cppcheck:
 
 # create scanner with re2c
 re2c: src/json.hpp.re2c
-	$(RE2C) --bit-vectors --nested-ifs --no-debug-info $< | $(SED) '1d' > src/json.hpp
+	$(RE2C) -W --bit-vectors --nested-ifs --no-debug-info $< | $(SED) '1d' > src/json.hpp
 
 # pretty printer
 pretty:

src/json.hpp.re2c

@@ -7388,10 +7388,10 @@ class basic_json
                 number        { return token_type::value_number; }
 
                 // string
-                quotation_mark  = [\"];
+                quotation_mark  = ["];
                 escape          = [\\];
-                unescaped       = [^\"\\\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0A\x0B\x0C\x0D\x0E\x0F];
-                single_escaped  = [\"\\/bfnrt];
+                unescaped       = [^"\\\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0A\x0B\x0C\x0D\x0E\x0F];
+                single_escaped  = ["\\/bfnrt];
                 unicode_escaped = [u][0-9a-fA-F]{4};
                 escaped         = escape (single_escaped | unicode_escaped);
                 char            = unescaped | escaped;