You can use the in keyword in Kotlin with any class that implements operator contains

when_in.kt

/**
 * Using in keyword with other classes in Kotlin.
 * ----------------------------------------------
 *
 *
 * You can provide an operator contains function for a class so
 * that you can use the in syntax in a when statement.
 */


/**
 * For example if you for some reason wanted to do a when
 * comparing against multiple regular expressions you could can add
 * the contain operator function to the Regex class as so:
 */
operator fun Regex.contains(string: String) = this.find(string) != null

// This makes it possible to check for a [String] in a [Regex]

/**
 * Then a function as such is valid as it checks for [String] in [Regex]
 */
fun platform(description: String) = when (description) {
    in Regex(".*android.*", RegexOption.IGNORE_CASE) -> println("Platform is Android!")
    in Regex(".*windows.*", RegexOption.IGNORE_CASE) -> println("Platform is Windows!")
    in Regex(".*ios.*", RegexOption.IGNORE_CASE) -> println("Platform is iOS!")
    else -> print("Don't know this platform??")
}

fun main(args: Array<String>) {

    println("Checking platform of Mozilla/5.0 (Android; Mobile; rv:40.0) Gecko/40.0 Firefox/40.0")
    platform("Mozilla/5.0 (Android; Mobile; rv:40.0) Gecko/40.0 Firefox/40.0")

    println("Checking platform of Mozilla/5.0 (iPhone; CPU iPhone OS 8_3 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) FxiOS/1.0 Mobile/12F69 Safari/600.1.4")
    platform("Mozilla/5.0 (iPhone; CPU iPhone OS 8_3 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) FxiOS/1.0 Mobile/12F69 Safari/600.1.4")

    println("Checking platform of Mozilla/5.0 (Windows NT x.y; rv:10.0) Gecko/20100101 Firefox/10.0")
    platform("Mozilla/5.0 (Windows NT x.y; rv:10.0) Gecko/20100101 Firefox/10.0")
}


/**
 * Here is a more complex example from the MoshiPack source code:
 */

operator fun Array<MsgpackFormatType>.contains(value: Byte) = this.any { value in it }

data class MsgpackFormatType(val tag: Byte, val maxSize: Long, val isFix: Boolean = false) {

    operator fun contains(value: Byte) = if (isFix) value in tag..tag+maxSize else value == tag
}

/**
 * This allows checking if a Byte is contained by an Array of MsgpackFormatType.
 * Since MsgpackFormatType is a complex data class, it also needs to check if the Byte
 * belongs to it. 
 * 
 * This allows for the following when ->
 */

fun doProperByteAction(c: Byte) {
    when (c) {
        in MsgpackFormat.ARRAY -> println("Begin array!")
        in MsgpackFormat.MAP -> println("Begin map!")
        in MsgpackFormat.STR -> println("Begin string!")
        else -> TODO()
    }
}


object MsgpackFormat {
    
    const val SIZE_8 = 255L
    const val SIZE_16 = 65535L
    const val SIZE_32 = 4294967295

    val FIX_STR = MsgpackFormatType(0xa0.toByte(), 31, isFix = true)

    val STR_8 = MsgpackFormatType(0xd9.toByte(), SIZE_8)
    val STR_16 = MsgpackFormatType(0xda.toByte(), SIZE_16)
    val STR_32 = MsgpackFormatType(0xdb.toByte(), SIZE_32)
    val STR = arrayOf(FIX_STR, STR_8, STR_16, STR_32)

    val FIX_ARRAY = MsgpackFormatType(0x90.toByte(), 15, isFix = true)
    val ARRAY_16 = MsgpackFormatType(0xdc.toByte(), SIZE_16)
    val ARRAY_32 = MsgpackFormatType(0xdd.toByte(), SIZE_32)
    val ARRAY = arrayOf(FIX_ARRAY, ARRAY_16, ARRAY_32)

    val FIX_MAP = MsgpackFormatType(0x80.toByte(), 15, isFix = true)
    val MAP_16 = MsgpackFormatType(0xde.toByte(), SIZE_16)
    val MAP_32 = MsgpackFormatType(0xdf.toByte(), SIZE_32)
    val MAP = arrayOf(FIX_MAP, MAP_16, MAP_32)
}