Heh, sadly I remembered from your post that you were saying you couldn't get $regerrstr to work (where in fact you said you had no idea what it's for).
Luckily it came up on IRC and after quick testing it's exactly what the name suggests: it returns the PCRE string version of the error code returned by the different PCRE regex function calls.

@Khaled: I don't know why $regerrstr is undocumented but it is pretty useful to know if a pattern fails because of an error or because there is no match found. Please document this identifier.

Looking for a good help channel about mIRC? Check #mircscripting @ irc.swiftirc.net