A validator class written in Kotlin can be used to validate the data. Eg. checks for valid email, phone, password, name, etc.

Validator.kt

//
// Copyright © 2018 Kalpesh Talkar. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// For support: https://gist.github.com/KalpeshTalkar/e6a50efd08a3d5d142e47da559936bb7
//

import android.support.design.widget.TextInputLayout
import android.util.Patterns
import android.widget.EditText
import java.util.regex.Pattern

/**
 * Created by Kalpesh on 23/01/18.
 *
 * This class is used to validate data like email, phone, password policy, etc.
 * It also sets error to the EditText or TextInputLayout of the EditText if needed.
 */
class Validator {

    companion object {

        // Default validation messages
        private val PASSWORD_POLICY = """Password should be minimum 8 characters long,
            |should contain at least one capital letter,
            |at least one small letter,
            |at least one number and
            |at least one special character among ~!@#$%^&*()-_=+|[]{};:'\",<.>/?""".trimMargin()

        private val NAME_VALIDATION_MSG = "Enter a valid name"
        private val EMAIL_VALIDATION_MSG = "Enter a valid email address"
        private val PHONE_VALIDATION_MSG = "Enter a valid phone number"

        /**
         * Retrieve string data from the parameter.
         * @param data - can be EditText or String
         * @return - String extracted from EditText or data if its data type is Strin.
         */
        private fun getText(data: Any): String {
            var str = ""
            if (data is EditText) {
                str = data.text.toString()
            } else if (data is String) {
                str = data
            }
            return str
        }

        /**
         * Checks if the name is valid.
         * @param data - can be EditText or String
         * @param updateUI - if true and if data is EditText, the function sets error to the EditText or its TextInputLayout
         * @return - true if the name is valid.
         */
        fun isValidName(data: Any, updateUI: Boolean = true): Boolean {
            val str = getText(data)
            val valid = str.trim().length > 2

            // Set error if required
            if (updateUI) {
                val error: String? = if (valid) null else NAME_VALIDATION_MSG
                setError(data, error)
            }

            return valid
        }

        /**
         * Checks if the email is valid.
         * @param data - can be EditText or String
         * @param updateUI - if true and if data is EditText, the function sets error to the EditText or its TextInputLayout
         * @return - true if the email is valid.
         */
        fun isValidEmail(data: Any, updateUI: Boolean = true): Boolean {
            val str = getText(data)
            val valid = Patterns.EMAIL_ADDRESS.matcher(str).matches()

            // Set error if required
            if (updateUI) {
                val error: String? = if (valid) null else EMAIL_VALIDATION_MSG
                setError(data, error)
            }

            return valid
        }

        /**
         * Checks if the phone is valid.
         * @param data - can be EditText or String
         * @param updateUI - if true and if data is EditText, the function sets error to the EditText or its TextInputLayout
         * @return - true if the phone is valid.
         */
        fun isValidPhone(data: Any, updateUI: Boolean = true): Boolean {
            val str = getText(data)
            val valid = Patterns.PHONE.matcher(str).matches()

            // Set error if required
            if (updateUI) {
                val error: String? = if (valid) null else PHONE_VALIDATION_MSG
                setError(data, error)
            }

            return valid
        }

        /**
         * Checks if the password is valid as per the following password policy.
         * Password should be minimum minimum 8 characters long.
         * Password should contain at least one number.
         * Password should contain at least one capital letter.
         * Password should contain at least one small letter.
         * Password should contain at least one special character.
         * Allowed special characters: "~!@#$%^&*()-_=+|/,."';:{}[]<>?"
         *
         * @param data - can be EditText or String
         * @param updateUI - if true and if data is EditText, the function sets error to the EditText or its TextInputLayout
         * @return - true if the password is valid as per the password policy.
         */
        fun isValidPassword(data: Any, updateUI: Boolean = true): Boolean {
            val str = getText(data)
            var valid = true

            // Password policy check
            // Password should be minimum minimum 8 characters long
            if (str.length < 8) {
                valid = false
            }
            // Password should contain at least one number
            var exp = ".*[0-9].*"
            var pattern = Pattern.compile(exp, Pattern.CASE_INSENSITIVE)
            var matcher = pattern.matcher(str)
            if (!matcher.matches()) {
                valid = false
            }

            // Password should contain at least one capital letter
            exp = ".*[A-Z].*"
            pattern = Pattern.compile(exp)
            matcher = pattern.matcher(str)
            if (!matcher.matches()) {
                valid = false
            }

            // Password should contain at least one small letter
            exp = ".*[a-z].*"
            pattern = Pattern.compile(exp)
            matcher = pattern.matcher(str)
            if (!matcher.matches()) {
                valid = false
            }

            // Password should contain at least one special character
            // Allowed special characters : "~!@#$%^&*()-_=+|/,."';:{}[]<>?"
            exp = ".*[~!@#\$%\\^&*()\\-_=+\\|\\[{\\]};:'\",<.>/?].*"
            pattern = Pattern.compile(exp)
            matcher = pattern.matcher(str)
            if (!matcher.matches()) {
                valid = false
            }

            // Set error if required
            if (updateUI) {
                val error: String? = if (valid) null else PASSWORD_POLICY
                setError(data, error)
            }

            return valid
        }

        /**
         * Sets error on EditText or TextInputLayout of the EditText.
         * @param data - Should be EditText
         * @param error - Message to be shown as error, can be null if no error is to be set
         */
        private fun setError(data: Any, error: String?) {
            if (data is EditText) {
                if (data.parent.parent is TextInputLayout) {
                    (data.parent.parent as TextInputLayout).setError(error)
                } else {
                    data.setError(error)
                }
            }
        }

    }

}

It would be so nice to give an example of how to use these functions, I tried with.

       val email = findViewById<EditText>(R.id.email)
        println(isValidEmail(email))

Usage:

// To validate email address:
// You can pass string or edit text as a parameter.
// If you need to show error on your edit text or text input layout, pass the second parameter as true else false.
Validator.isValidEmail(YOUR_EDIT_TEXT_OR_STRING, SHOW_ERROR_FLAG)

// Below code checks if the passed string is a valid email or not.
Validator.isValidEmail("name@domain.com", false)

// Below code checks if the passed edit text has a valid email or not.
// If the email is not valid, error will be show on the edit text.
Validator.isValidEmail(etEmail, true)

It would be so nice to give an example of how to use these functions, I tried with.

       val email = findViewById<EditText>(R.id.email)
        println(isValidEmail(email))

@cyberespia Thank you for having a look at my gists.
I have shown usage example in my comment.

IsValidPassword always returns true (line 180).
should be

return valid

IsValidPassword always returns true (line 180).
should be

return valid

@elijellyeli Oops! my bad. Thanks for pointing out the error. Updated the file.

how to use

how to use

Please check the comments above to see the usage and example.