This is how to represent an object that can have multiple types within a JSON Schema

multi-type-schema.json

{
   "$schema": "http://json-schema.org/draft-03/schema#",
   "type": [
     {
        "type": "number",
        "minimum": 0,
        "exclusiveMinimum": true
     },
     {
        "type": "string",
        "pattern": "^.+@.+\\..+$"
     }
   ]
 }

Using "oneOf" also gives more of a hint for automated processors, as well as human readers. For example, if you paste this schema:

{ 
    "oneOf": [ 
        {
            "title": "Number",
            "type": "number", 
            "minimum": 0, 
            "exclusiveMinimum": true 
        }, 
        {
            "title": "String",
            "type": "string"
        } 
    ] 
}

into this page (selecting the "Editable" option), then the interface gives you a nice drop-down for the different schema options.

However, if you replace "oneOf" with "anyOf", then the interface doesn't know the schemas are exclusive, so it presents a multiple-select instead (hidden behind a button), which isn't quite as intuitive.

@geraintluff thanks for your input, that is more intuitive especially in this context.