I'm not seeing a way for custom transformations

[KegenGuyll]

Problem

I'm wanting to see if a value exist on the page web page. If the value exist then return true else false.

Different Attempts

1. I tried matching regex to see if this would work, but it always returns false even if the value matches.

{
      "name": "Master Card",
      "baseSelector": "#aspnetForm > table",
      "fields": [
      // ... other fields that are not relative
      {
      "name": "is_rookie",
      "selector": "#ctl00_MainContent_cellMiddle > font:nth-child(10)",
      "type": "regex",
      "pattern": r"Rookie",
      "default": False
      }
      ]
    }

2. Custom transformer, I figured this solution would be the most straight forward to get working. Mostly because the documentation says I can do this.
   
   https://docs.crawl4ai.com/extraction/no-llm-strategies/

But in practice, I don't see any example of how to do this (maybe I'm doing something wrong). I've looked at the function that handles the transform and it doesn't even look possible to pass your own transform functions.

https://github.com/unclecode/crawl4ai/blob/3b1025abbb6e2565602c05f9a959458da3531f3a/crawl4ai/extraction_strategy.py#L991C4-L1014C21

I'm kinda stuck here and would love some input on possible solutions and if it would prove helpful I can post more snippets of my code.

There is more that likely some obvious solution, that is staring me right in the face and I'm too dumb to notice it.