Why do regex patterns result in "bad string" errors in jsonschema?

I'm using a regex to validate a property, in json schema.

The schema is as follows:

{
    "$schema": "http://json-schema.org/draft-07/schema",
    "$id": "http://example.com/example.json",
    "type": "object",
    "required": [
        "library_version"
    ],
    "properties": {
        "library_version": {
            "$id": "#/properties/library_version",
            "type": "string",
            "title": "Library version",
            "description": "The library version (e.g. 0.0.1) used to create this file.",
            "pattern": "^(0|[1-9]\d*)\.(0|[1-9]\d*)\.(0|[1-9]\d*)(?:-((?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\+([0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?$",
            "message": {
                "required": "Library version is a required property",
                "pattern": "The version must be a semantic form, like 0.0.1 (see https://semver.org/)"
            },
            "examples": [
                "0.0.1"
            ]
        }
    }
}

Where the big regex is for a semver - c.f. here.

Using jsonlint I get:

Error: Parse error on line 14:
...le.",            "pattern": "^(0|[1-9]\d*)\.(0|[
----------------------^
Expecting 'STRING', 'NUMBER', 'NULL', 'TRUE', 'FALSE', '{', '[', got 'undefined'

Surely I don't have to escape regex patterns in json schema? Strings should be treated as literals, right? Does anyone know what on earth is going on here?