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The propertyNames property makes it possible to define rules that specify the property names that are allowable within the JSON Instance object.

"MyObject": {
    "type": "object",
    "additionalProperties": {
        "type": "string"
    },
    "propertyNames": {
        "type": "string",
        "pattern": "VAL_\\d+"
    }
},

In this example "MyObject" is describes an object that can contain string properties with any name. The Property Names rules imposes a limitation on the property names, specifying that they must match the Pattern (regular expression) "VAL_\d+".

"MyObject": {
    "VAL_1" : "My Value",
    "VAL_4" : "My Value",
    "VAL_0001" : "",
    "VAL_1" : "Some other value"
}

The functionality is very similar to Pattern Properties, but allows more control and can be easier to read.