You can also do:

regsubex version:

alias a4 { return $regsubex($1,/\[[^]]+\]/g,) }

regsub version:

alias a4 { var %r | noop $regsub($1,/\[[^]]+\]/g,,%r) | return %r }

.echo -q can be used in place of noop if someone uses an older copy of mirc that doesn't recognize noop command.