Filter a substring in kotlin

Question

In kotlin I'd like to filter a string and return a substring of only valid characters. Say we have valid characters,

valid = listOf('A', 'B', 'C')

How can I define a fcn in kotlin in the most succinct way to filter a string and only retain valid characters? For example,

'ABCDEBCA' --> 'ABCBCA'
'AEDC'     --> 'AC'

Having trouble finding a canonical way to do this without resorting to using an array of string.

import kotlin.text.filter

class Test(){
    val VALID = listOf("A", "B", "C")

    fun filterString(expression: String): String{
         expression.filter(x --> !VALID.contains(x)) #Doesn't work
    }
}

The filter docs doesn't show any examples specifically for spring manipulation.

Answer 1

val VALID = setOf('A', 'B', 'C') // lookup in a set is O(1), whereas it's O(n) in a list. The set must contain Chars, not Strings
val expression = "ABCDEFEDCBA"
val filtered = expression.filter { VALID.contains(it) }
println(filtered)
// ABCCBA

Or

val VALID = setOf('A', 'B', 'C')

fun filterString(expression: String) = expression.filter { it in VALID }

fun main(args: Array<String>) {
    val expression = "ABCDEFEDCBA"
    val filtered = filterString(expression)
    println(filtered)
    // ABCCBA
}