Right, my bad, $regsubex is eating the $N- tokens, I thought it was only doing so when the subtext contains special markers, but it always does it, using $regsub works better then:

Code
alias reg_acro {
  var -p %p $1-
  noop $hfind(reg_acro_table,*,0,w,noop $regsub(%p,$+(/\b\Q,$replacecs($1,\E,\E\\E\Q),\E\b/),$hget(reg_acro_table, $1),%p))
  return $+($upper($left(%p,1)),$mid(%p,2))
}

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