DigitsTrimStartFilter.kt

class DigitsTrimStartFilter(private val max: Int) : InputFilter {
   override fun filter(...): CharSequence? {
       val s = source.subSequence(start, end).filter { it.isDigit() }
       val keep = max - (dest.length - (dend - dstart))
       return when {
           keep <= 0 -> ""
           keep >= s.length -> null // keep original
           else -> s.subSequence(s.length - keep, s.length)
       }
   }
}