Huh. Well egg on my face.
Though having $regsubex() output the match count, as $regsub() does, would be useful to know for if-conditions and whether any substitutions were made.
Additionally, the ability to input/output binary variables directly to and from $regsub() and $regsubex() would make using /parseline and /sockwrite a whole lot easier. It's often unacceptable for these commands to collapse spaces, which means a lot of &binvar use, which means a lot of switching back and forth between binary variables and space-placeholder text in order to make any string manipulations.
var %parseline = $regsubex($parseline,...)
var %p = $replace(%parseline,$chr(32),$chr(01))
bset -t &b 1 %p
breplace &b 01 32
parseline -ibn &b