I gave up trying to deal with the interface, so I just grepped "regex" in my splunk folder instead and found what I was looking for. I'm sure there's a much simpler way to do this.
I was looking around and I found the answer here: http://answers.splunk.com/answers/36296/how-to-edit-or-delete-a-custom-field.html
"In Splunk Web, you navigate to the Field extractions page by selecting Manager > Fields > Field extractions."
- Answer by lihong007