How to use show set to select non text characters in math equation?

Questions
9 
#show math.equation: it => {
  show regex(".+"): it => {
    if it.text in ("a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"){
      return text(it, red)
    }
    it
  }
  it
}
$"ABCDabcd" a b c d$