I am using this regex pattern to allow only valid characters. "^[a-z_A-Z0-9 ]*$";

Here is the java code

package com.valid;

import java.util.regex.Matcher;

import java.util.regex.Pattern;

import android.app.Activity;

import android.os.Bundle;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

import android.widget.EditText;

import android.widget.Toast;

public class ValidationActivity extends Activity {

    EditText tbox;

    Button but;

    @Override

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);

        tbox = (EditText)findViewById(R.id.EditText01);

        but  = (Button)findViewById(R.id.Button01);

        but.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {

                if(isValid(tbox.getText().toString().trim())){

                    Toast.makeText(getApplicationContext(), "Text entered is OK.", Toast.LENGTH_SHORT).show();

                 }else{

                    Toast.makeText(getApplicationContext(), "Oops!! Text entered is Invalid.", Toast.LENGTH_SHORT).show();

                }

            }

        });

    }

    public static boolean isValid(String str)

    {

            boolean isValid = false;

            String expression = "^[a-z_A-Z0-9 ]*$";

           /**This interface represents an ordered set of characters and defines the methods to probe them.**/

            CharSequence inputStr = str;

            Pattern pattern = Pattern.compile(expression);

            Matcher matcher = pattern.matcher(inputStr);

            if(matcher.matches())

            {

                isValid = true;

            }

            return isValid;

     }

}

main.xml

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

    android:orientation="vertical"

    android:layout_width="fill_parent"

    android:layout_height="fill_parent"

    >

<TextView

    android:layout_width="fill_parent"

    android:layout_height="wrap_content"

    android:text="@string/hello"

    />

<EditText

    android:text=""

    android:hint="enter some text"

    android:id="@+id/EditText01"

    android:layout_width="fill_parent"

    android:layout_height="wrap_content">

</EditText>

<Button

    android:text="Test"

    android:id="@+id/Button01"

    android:layout_width="wrap_content"

    android:layout_height="wrap_content">

</Button>

</LinearLayout>

Snapshot:-