I always used to be a 'regexp' person, but prolonged exposure to 'regex' seems to have recently built up enough activation energy to switch me over.

Now I'm on the other side, I find myself wondering what the point of the p was in the first place – 'regex' is adequately unambiguous and takes fewer characters, so what's not to like? But I can't remember why I adopted the p spelling myself. I'd be quite interested to know whether there are any centralised sources (particular textbooks, library APIs, influential articles or some such) that might be responsible for popularising one or the other.

The perl manpages use regexp.

It's nicer when both bits of the abbreviation are the same length?

I hadn't thought of that, though the thought has just occurred to me that 'exp' by itself, shorn of the 'reg' prefix, makes a more comprehensible abbreviation of 'expression' than 'ex' does.

I was thinking that. I think I use regexp occasionally but regex more. I *think* I learned regexp first, but it eroded quickly, but I'm not sure, it could have been the other way round.

I know that python, Java and C# prefer "regex", JavaScript prefers "regexp", R is inconsistent.

Perl (at least "man perlre") prefers "regexp".