Negative lookbehind pattern causes validation to fail

[richardscholten73]

Hello,

When adding a negative lookbehind to a regular expression in my field validation (.net core api with swashbuckle swaggerui) the resulting swagger json is marked with an "invalid" badge.
But the regexp pattern is working just fine and the swagger seems to be valid.
Even the swagger online editor is not marking it as invalid.

The problem lies in the "pattern": "^(?<![\/\. ])$"
I narrowed it down to the "<" character. removing that the json swagger validates.
of course removing it is not a solution because i need it for the negative lookbehind.

The swagger json:

{
  "openapi": "3.0.1",
  "info": {
    "title": "API",
    "version": "0.1"
  },
  "paths": {
    "/api/v0.1/files": {
      "delete": {
        "tags": [
          "Files"
        ],
        "summary": "Delete",
        "parameters": [
          {
            "name": "path",
            "in": "query",
            "description": "",
            "schema": {
              "pattern": "^(?<![\\/\\. ])$",
              "type": "string"
            }
          }
        ],
        "responses": {
          "204": {
            "description": "Success"
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
    }
  }
} 

The message:

{
  schemaValidationMessages: [
    {
      level: "error",
      domain: "validation",
      keyword: "oneOf",
      message: "instance failed to match exactly one schema (matched 0 out of 2)",
      schema: {
        loadingURI: "#",
        pointer: "/definitions/Operation/properties/parameters/items"
      },
      instance: {
        pointer: "/paths/~1api~1v0.1~1files/delete/parameters/0"
      }
    }
  ]
}

kind regards,
Richard