Do Grok patterns have a place in the if =~ conditional matching?

I have the case where..

Sometimes my log has a block of JSON.
Sometimes that block of JSON has a field (Lets call it "IPAddress")
Sometimes the field "IPAddress" has a valid IP, other times, I've seen it with the value "unknown" or an empty string.

I want to..

If the "IPAddress" field exists..
If the field contains a valid IP address..

THEN, apply the geoip filter to it.

I know that there is a %{IP} defined pattern for Grok.
What I would like to do is something similar to..

if [IPAddress] =~ %{IP} {
  #Do stuff

However there are few caveats I've seen.

  1. It doesn't seem to work that way. In place of that, I found the base regex that comprises the %{IP} pattern, and now have done this:
    if [IPAddress] =~ /Long horrible string of regex.. seriously like 8 lines/
    That works. But, its messy.

  2. What kind of performance impact does this present, if any? And would any performance be gained by replacing the long regex string with %{IP}?

So overall, I would argue that a more readable config file, using Grok patterns for situations like this, is a benefit.
What do you guys think?

And, maybe this is just a horrible way to approach my problems. What might be some other ways to solve this?

Edit: Post had multiple items as a numbered list... all with the same number. Fixed. Also changed some words and emphasized some text for clarity

It doesn't seem to work that way.

Yeah, I'm pretty sure the regexps in conditionals don't support grok patterns.

And would any performance be gained by replacing the long regex string with %{IP}?

I'd assume the IP pattern in the end expands to more or less the same expression that you're currently using, so no.

But why have such a strict regexp? Is it really likely that the IPAddress field frequently will contain strings that match a simple expression like ^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$ yet aren't valid IPv4 addresses?

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So.. that's definitely a valid question.
Originally I was trying to match with a grok filter, using %{IP}. I carried that over when I ended up making it an IF statement.

My thinking is.. if I am going to end up matching a field with %{IP} at any point, I would want my if statement to be an exact replica of that match. Not leaving any gaps. I didn't have any specific examples in mind, it was just a way to not leave any ground open for unexpected errors in the future.