Real-time feedback about user input using regex and RxKotlin 2.x for "set it and forget it" in onCreate

InputActivity.kt

import android.graphics.Color
import android.text.Editable
import android.text.TextWatcher
import android.widget.EditText
import io.reactivex.subjects.PublishSubject
import io.reactivex.Observable
import io.reactivex.rxkotlin.Observables

class InputActivity {

    private lateinit var tickerName: EditText
    private lateinit var tickerPrice: EditText
    private lateinit var tickerObservable: Observable<String>
    private lateinit var targetObservable: Observable<String>
    private lateinit var isUserInputNotEmpty: Observable<Boolean>
    private lateinit var tickerHelper: TextView
    private lateinit var targetHelper: TextView

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.input_activity)

        tickerName = findViewById(R.id.tickerName)
        tickerPrice = findViewById(R.id.tickerPrice)
        tickerHelper = findViewById(R.id.tickerHelper)
        targetHelper = findViewById(R.id.targetHelper)
        tickerObservable = getTextWatcherObservable(tickerName)
        targetObservable = getTextWatcherObservable(tickerPrice)

        fun onNextTickerValidate(t: String) {
            if (t.isEmpty()) {
                tickerHelper.text = "Empty"
                tickerHelper.setTextColor(Color.MAGENTA)
            } else {
                if (!t.isValidTickerSymbol()) {
                    tickerHelper.text = "Malformed"
                    tickerHelper.setTextColor(Color.RED)
                } else {
                    tickerHelper.text = "OK"
                    tickerHelper.setTextColor(Color.GREEN)
                }
            }
        }
        fun onNextTargetBetterNotBeEmpty(p: String) {
            if (p.isEmpty()) {
                targetHelper.text = "Empty"
                targetHelper.setTextColor(Color.RED)
            } else {
                targetHelper.text = "OK"
                targetHelper.setTextColor(Color.GREEN)
            }
        }
        tickerObservable.subscribe({ t -> onNextTickerValidate(t) })
        targetObservable.subscribe({ p -> onNextTargetBetterNotBeEmpty(p) })
    }

    private fun getTextWatcherObservable(editText: EditText): Observable<String> {
        val subject = PublishSubject.create<String>()

        editText.addTextChangedListener(object : TextWatcher {
            override fun beforeTextChanged(s: CharSequence, start: Int, count: Int, after: Int) { }
            override fun onTextChanged(s: CharSequence, start: Int, before: Int, count: Int) { }
            override fun afterTextChanged(s: Editable) {
                subject.onNext(s.toString())
            }
        })

        return subject
    }
    
    fun String.isValidTickerSymbol(): Boolean {
        val validTickerFormat =
                Regex("^(([a-z]{2,4}):(?![a-z\\d]+\\.))?([a-z]{1,4}|\\d{1,3}(?=\\.)|\\d{4,})(\\.([a-z]{2}))?$",
                        RegexOption.IGNORE_CASE)

        val results = validTickerFormat.matchEntire(this)?.groupValues

        return (results != null)
    }
}