What is "it" that should be properly documented?

Regular expressions are not a syntax of mIRC, they are a syntax of the PCRE library that mIRC uses. The only thing mIRC does not currently document is that mIRC uses PCRE, though this would likely not change the fact that commas are used in expressions.

Since regexes are not syntax, the expressions themselves are nothing more than string input, like everything else in the language. Regex string input has the same caveats as any other kind of string input when used in $identifiers, where arguments delimit across ",". Using /\d{3,4}/ in an identifier is no different than using:

Code:
//echo -a $len(Hello, how are you?)


If you understand why the above fails, you should understand why regexes fail.