In thread in #17208, @Managor brought up the good argument that we should standardise references to Regular Expresisons. Per data shared by @dmmqz, it looks like:

At the moment "regex" and "regular expression" is the most used (94 and 98 occurrences respectively). "Regex" appears once and "REGEX" twice.

Accordingly, I propose we use regex universally and define this in the style guide. I can update the style guide if we move forward with this as well as go through our content and fix instances of "regular expression", "Regex", and "REGEX", and this is also a good first issue style effort.