Various utility functions to validate user input.

InputValidationUtils.java

package com.example.util;

import android.text.TextUtils;
import android.util.Patterns;

import androidx.annotation.NonNull;

import java.util.Objects;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Various utility functions to validate user input.
 *
 * @author Abdallah Abdelazim
 */
public final class InputValidationUtils {

    private InputValidationUtils() {
        // Prevent instantiation
    }

    /**
     * Checks that the passed string is a valid email address.
     *
     * @param email the string to validate.
     * @return true if {@code s} contains a valid email address, false otherwise.
     */
    public static boolean isEmailValid(String email) {
        return (!TextUtils.isEmpty(email) && Patterns.EMAIL_ADDRESS.matcher(email).matches());
    }

    /**
     * Checks if a given string contains any special characters.
     * <br>
     * A special character is any character that is not A-Z, a-z, 0-9 or any whitespace.
     *
     * @param s the string to validate.
     * @return true if {@code s} contains one or more special characters,
     * false if it contains no special characters at all.
     */
    public static boolean containsSpecialChar(String s) {
        return isRegexMatch("[^a-zA-Z0-9\\s]", s);
    }

    /**
     * Checks if a given string starts with a special character.
     * <br>
     * A special character is any character that is not A-Z, a-z, 0-9 or any whitespace.
     *
     * @param s the string to validate.
     * @return true if the string starts with a special character while ignoring all
     * leading whitespaces, false otherwise.
     */
    public static boolean isStartingWithSpecialChar(String s) {
        return isRegexMatch("^\\s*[^a-zA-Z0-9\\s](.|\\s)*$", s);
    }

    /**
     * Checks if a given string contains only special characters.
     * <br>
     * A special character is any character that is not A-Z, a-z, 0-9 or any whitespace.
     *
     * @param s the string to validate.
     * @return true if all the non-whitespace characters in the string are special characters,
     * false otherwise.
     */
    public static boolean isAllSpecialChars(String s) {
        return isRegexMatch("^(\\s*[^a-zA-Z0-9\\s]\\s*)+$", s);
    }

    /**
     * Checks if a given string starts with an invalid character.
     * <br>
     * An invalid character is any punctuation or control character.
     *
     * @param s the string to validate.
     * @return true if the string starts with an invalid character while ignoring all
     * leading whitespaces, false otherwise.
     */
    public static boolean isStartingWithInvalidChar(String s) {
        return isRegexMatch("^\\s*[\\p{Punct}\\p{Cntrl}](.|\\s)*$", s);
    }

    /**
     * Checks if a given string contains only invalid characters.
     * <br>
     * An invalid character is any punctuation or control character.
     *
     * @param s the string to validate.
     * @return true if all the non-whitespace characters in the string are invalid characters,
     * false otherwise.
     */
    public static boolean isAllInvalidChars(String s) {
        return isRegexMatch("^(\\s*[\\p{Punct}\\p{Cntrl}]\\s*)+$", s);
    }

    private static boolean isRegexMatch(@NonNull String regex, String s) {
        if (s == null || s.isEmpty())
            return false;

        Pattern pattern = Pattern.compile(Objects.requireNonNull(regex,
                "The passed regex string cannot be null"));

        Matcher matcher = pattern.matcher(s);

        return matcher.matches();
    }
}