How can I validate EditText input in Android using Kotlin?

This example demonstrates how to validate EditText input in Android using Kotlin.
Step 1 − Create a new project in Android Studio, go to File ⇒ New Project and fill all required details to create a new project.
Step 2 − Add the following code to res/layout/activity_main.xml.
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   tools:context=".MainActivity">
   <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_centerHorizontal="true"
      android:layout_marginTop="70dp"
      android:background="#008080"
      android:padding="5dp"
      android:text="TutorialsPoint"
      android:textColor="#fff"
      android:textSize="24sp"
      android:textStyle="bold" />
   <EditText
      android:id="@+id/editText"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_centerInParent="true"
      android:hint="Enter alpha-numeric text"
      android:inputType="text"
      tools:ignore="Autofill" />
   <TextView
      android:id="@+id/textView"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_below="@+id/editText"
      android:layout_centerHorizontal="true"
      android:layout_marginTop="30dp"
      android:textColor="@android:color/holo_orange_dark"
      android:textSize="24sp"
      android:textStyle="bold" />
</RelativeLayout>
Step 3 − Add the following code to src/MainActivity.kt
import android.os.Bundle
import android.text.InputFilter
import android.text.Spanned
import android.widget.EditText
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity
import kotlinx.android.synthetic.main.activity_main.*
import java.util.regex.Matcher
import java.util.regex.Pattern
class MainActivity : AppCompatActivity() {
   private lateinit var textView: TextView
   override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.activity_main)
      title = "KotlinApp"
      val editText: EditText = findViewById(R.id.editText)
      textView = findViewById(R.id.textView)
      editText.filters = arrayOf<InputFilter>(ValidateFilter())
   }
   inner class ValidateFilter : InputFilter {
      override fun filter(source: CharSequence, start: Int, end: Int, dest: Spanned?, dstart: Int, dend: Int): CharSequence? {
         for (i in start until end) {
            val string = editText.text.toString().trim()
            val checkMe = source[i].toString()
            val pattern: Pattern = Pattern.compile("   [ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz123456789_]*")
            val matcher: Matcher = pattern.matcher(checkMe)
            val valid: Boolean = matcher.matches()
            if (!valid) {
               textView.text = string
            }
         }
         return null
      }
   }
}
Step 4 − Add the following code to androidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.example.q11">
   <application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:roundIcon="@mipmap/ic_launcher_round"
      android:supportsRtl="true"
      android:theme="@style/AppTheme">
      <activity android:name=".MainActivity">
         <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
         </intent-filter>
      </activity>
   </application>
</manifest>
Let's try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from android studio, open one of your project's activity files and click the Run icon from the toolbar. Select your mobile device as an option and then check your mobile device which will display your default screen.