matchloop a, b
{ do something }    

macro matchloop x& {
  local t, s
  macro t \{
    macro t \\{
      match x \\\{
        s
        t
      \\\}
    \\}
    t
  \}
  t
  macro s
}    

Do you mean a recursive macro?

If so, you can see the "allow_nesting" macro in win32ax.inc for that. It basically redefines itself within itself to get the recursion.

recursive macro

the "allow_nesting" macro in win32ax.inc

recursive macro xxx {xxx }    

macro fun a, b
  if (a)=0
    repeat 1 t:b
      display `t, 13, 10
    end repeat
  else
    fun (a)-1, (b)*(a)
  end if
end macro
fun 10, 1

macro mov op1, op2
  display 'mov ', `op1, ',', `op2, 13, 10
end macro
macro mov op1, op2
  if `op1 eq 'ip'
    jmp op2
  else
    mov op1, op2
  end if
end macro
    
mov ax, cx    

3628800
mov ax,cx