Escaping backslash in grok pattern giving error

[pk.241011]

This is the message I am working with:

C:\Data\Projects\Demo\2019-03-06-14-35-07_check.log

In the grok debugger this was the pattern I checked and it worked.

%{PATH:dontwant}\\%{USERNAME:identifier}_%{GREEDYDATA:drop} 

Output:

{
  "drop": "check.log",
  "identifier": "2019-03-06-14-35-07",
  "dontwant": "C:\\Data\\Projects\\Demo"
}

Now I wanted to put this in my pipeline. But I am seeing errors.

For this:

{
      "grok": {
        "field": "FileName",
        "patterns": ["%{PATH:dontwant}\\%{USERNAME:identifier}_%{GREEDYDATA:drop}"]
      }
    },

I get error:

{
  "error": {
"root_cause": [
  {
    "type": "parse_exception",
    "reason": "[patterns] Invalid regex pattern found in: [%{PATH:dontwant}\\%{USERNAME:identifier}_%{GREEDYDATA:drop}]. unmatched close parenthesis",
    "header": {
      "processor_type": "grok",
      "property_name": "patterns"
    }
  }
],
"type": "parse_exception",
"reason": "[patterns] Invalid regex pattern found in: [%{PATH:dontwant}\\%{USERNAME:identifier}_%{GREEDYDATA:drop}]. unmatched close parenthesis",
"header": {
  "processor_type": "grok",
  "property_name": "patterns"
}
  },
  "status": 400
}

For this:

{
      "grok": {
        "field": "FileName",
        "patterns": ["%{PATH:dontwant}\%{USERNAME:identifier}_%{GREEDYDATA:drop}"]
      }
    },

I get error:

{
  "error": {
    "root_cause": [
      {
        "type": "parse_exception",
        "reason": "Failed to parse content to map"
      }
    ],
    "type": "parse_exception",
    "reason": "Failed to parse content to map",
    "caused_by": {
      "type": "i_o_exception",
      "reason": "Unrecognized character escape '%' (code 37)\n at [Source: org.elasticsearch.common.bytes.BytesReference$MarkSupportingStreamInputWrapper@1b5eba15; line: 33, column: 41]"
    }
  },
  "status": 400
}

Any pointers on how I can make it work?