Steps to reproduce:

1. Create value map using example

   {
       "hostid": 10416,
       "name": "API test",
       "mappings": [
           {
               "type": 4,
               "value": "@invalid regex [",
               "newvalue": "test"
           }
       ]
   }
   

Result:
No errors, value map is created.
Expected:
Error message "Invalid parameter "/1/mappings/1/value": invalid regular expression." is returned