As the title says, what is the correct plural form of regex?
To my knowledge, regex is an abbreviation for regular expression.
I see that some sources suggest regexes and regexen are both correct. The latter plural form sounds maybe German?
This is domain-specific jargon (the domain being Information Technology), rather than standard English, and thus may not conform to the rules. Wiktionary and Quora both give regexes or regexps; while I don’t find regexen unlikely, I’ve not actually seen/heard it used, and it would not come to mind immediately for me.
(You are correct that it is an abbreviation/shorthand for regular expression)