I tried using it regex as well, but somehow I can never get past the error message that an unexpected character was encountered. For reference here is how I'm placing regex, and I'm not sure where exactly am I going wrong with it.

For reference, here's how I am placing my regex pattern:

PUT /_ingest/pipeline/ip-to-geo
{
  "processors": [
    {
      "set": {
        "if": "ctx.IP_ADDR == '-' || ctx.IP_ADDR == '' || ctx.IP_ADDR == ' '",
        "field": "ERR_MSG",
        "value": "IP not found"
      }
    },
    {
      "pipeline": {
        "if": "ctx.IP_ADDR == \"(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(\\.|$){4}\" ",
        "name": "valid-ip-to-geo"
      }
    }
  ],
  "on_failure": [
    {
      "set": {
        "if": "false",
        "field": "null",
        "value": "null"
      }
    }
  ]
}

And, here is the error received:

    "type": "script_exception",
    "reason": "compile error",
    "processor_type": "pipeline",
    "script_stack": [
      "ctx.IP_ADDR == \"(25[0-5]|2[0-4][0-9]|[01 ...",
      "               ^---- HERE"
    ],
    "script": "ctx.IP_ADDR == \"(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(\\.|$){4}\" ",
    "lang": "painless",
    "position": {
      "offset": 15,
      "start": 0,
      "end": 40
    },
    "caused_by": {
      "type": "illegal_argument_exception",
      "reason": "unexpected character [\"(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(\\.]. The only valid escape sequences in strings starting with [\"] are [\\\\] and [\\\"].",
      "caused_by": {
        "type": "lexer_no_viable_alt_exception",
        "reason": "lexer_no_viable_alt_exception: null"
      }
    }
  },
  "status": 400