What you are asking for can be done with a simple while loop, but the first regex I gave does the exact same thing, just in the background where you can't see it.

alias firstletter1 {
  noop $regex($1,/([a-z])/i)
  return $regml(1)

alias firstletter2 { 
  var %i = 0, %ii = $len($1)
  while (%i < %ii) {
    inc %i
    if ($mid($1,%i,1) isalpha) return $v1

Both of those codes should return the same thing.