Code
alias reg_acro {
  var -p %p $1-
  noop $hfind(reg_acro_table,*,0,w,%p = $regsubex(%p,$+(/\b\Q,$replacecs($1,\E,\E\\E\Q),\E\b/),$hget(reg_acro_table,$1)))
  return $+($upper($left(%p,1)),$mid(%p,2))
}
Not tested but should work.
The original code had several issue and was poorly performing, this fix the issue and is also much faster, you need one of the latest mIRC version.

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