I've noticed some weirdness in the behaviour of the $regsub identifier. The items matched in parantheses can be used with \1 \2 etc but mirc identifiers fail to treat them as numbers. For example:
Code:
  
//set %decode hiya #23 wat #40 up | var %t = $regsub(%decode, /#([a-fA-F0-9]{2})/g, $iif(\1,$ifmatch), %decode) | echo -a %decode

echos "hiya 23 wat 40 up"

Code:
 
//set %decode hiya #23 wat #40 up | var %t = $regsub(%decode, /#([a-fA-F0-9]{2})/g, $iif(\1,$calc($ifmatch + 1)), %decode) | echo -a %decode

echos "hiya 0 wat 0 up"

$calc does not recognise it as a numeric value.

Any identifier that does something with a number fails as if it were null or just random text. For example $chr(\1) doesn't work, but string identifiers like $len, $+ etc handle it with no problems.