Regex Replace Issue

Hello,

I'm trying to do a regex replace to grab a combination of letters and digits but throw out the rows where I don't get anything.

Here is my formula:

case when REGEXP_LIKE(`Asset name`,'^.*([a-z]{4,5}\d{6}).*$')=1 THEN

REGEXP_REPLACE(`Asset name`,'^.*stc=?([a-z]{4,5}\d{6}).*$','$1')

else NULL end

To explain, I'm trying to pick up 4 or 5 letters after stc=(= sign is optional hence the '?') followed then by 6 digits. It is working for a lot of my data, but then I'm getting lots of rows that are picking up a pattern and showcasing it below in STC when it really should be blank. Because as you can see it shows stc and then no letters and therefore should be NULL. I have tested this pattern and this example on multiple regex helper sites and they all show that there should be no match. I don't know why domo might be picking this up.