regex101.com and regexbuddy.com (as provided by @bowesmana ) are both sites which provide ways of testing regular expressions (regex)
In Splunk, the rex and regex commands both use regular expressions (as do other functions in Splunk). Whether you want rex or regex, both the sites mentioned are useful tools for working out what your particular regex should be.
What is the pattern? Please describe it in more detail. (Regular expressions work by finding patterns but you have to be able to precisely describe the pattern.)
