After upgrading to PHP 7.0 my contact form outputs error

Upgraded the site from PHP 5.6 to PHP 7.0. After upgrading there was a code error with deprecated eregi(), which I replaced with preg_match() later, but now it outputs error every time I submit a the filled form.

It’s probably obvious, but I’m newbie at PHP, just learning.

That’s the full code of my contact form:

<?php
    /*
    Template Name: Обратная связь
    */

    if(isset($_POST['submitted'])) {
        if(trim($_POST['contactName']) === '') {
            $nameError="Please enter your name.";
            $hasError = true;
        } else {
            $name = trim($_POST['contactName']);
        }
        if(trim($_POST['email']) === '')  {
            $emailError="Please enter your email address.";
            $hasError = true;
        } else if (!preg_match("/^[A-Z0-9.%-]+@[A-Z0-9.%-]+.[A-Z]{2,4}$/", trim($_POST['email']))) {
            $emailError="You entered an invalid email address.";
            $hasError = true;
        } else {
            $email = trim($_POST['email']);
        }
        if(trim($_POST['comments']) === '') {
            $commentError="Please enter a message.";
            $hasError = true;
        } else {
            if(function_exists('stripslashes')) {
                $comments = stripslashes(trim($_POST['comments']));
            } else {
                $comments = trim($_POST['comments']);
            }
        }
        if(!isset($hasError)) {
            $emailTo = get_option('tz_email');
            if (!isset($emailTo) || ($emailTo == '') ){
                $emailTo = get_option('admin_email');
            }
            $subject = "Отзыв от пользователя ".$name;
            $body = "Имя: $name 
    E-mail: $email 

    $comments";
            $headers="From: ".$name.' <'.$emailTo.'>' . "rn" . 'Reply: ' . $email;

        mail($emailTo, $subject, $body, $headers);
            $emailSent = true;
        }
    } ?>

    <?php get_header(); ?>
    <div class="row">
    <div class="paper">
    <div class="paper-body">
                            <?php if(isset($emailSent) && $emailSent == true) { ?>
       <h2 class="contact-form-thanks">
          Спасибо, ваше сообщение отправлено.
       </h2>
    <?php } else { ?>
    <?php if(isset($hasError) || isset($captchaError)) { ?>
       <p class="error">Извините, произошла ошибка.<p>
    <?php } ?>
    <form action="<?php the_permalink(); ?>" id="contactForm" method="post">

            <input class="contact-form-name" type="text" name="contactName" id="contactName" value="" placeholder="Имя" required>

              <input class="contact-form-email" type="email" name="email" id="email" value="" placeholder="E-mail" required>

           <textarea class="contact-form-text" name="comments" id="commentsText" rows="10" cols="30" value="" placeholder="Сообщение" required></textarea>

           <button type="submit">Отправить</button>
    <button type="reset">Очистить</button>
        <input type="hidden" name="submitted" id="submitted" value="true">
    </form>
    </div>
    </div>
    <?php } ?>
                    </div><!-- .entry-content -->

    <?php get_footer(); ?>

WordPress has a built in function for validating emails and I would advise you use that.

So you can replace your preg_match with the wordpress is_email

else if (!preg_match("/^[A-Z0-9.%-]+@[A-Z0-9.%-]+.[A-Z]{2,4}$/", trim($_POST['email']))) {
            $emailError="You entered an invalid email address.";
            $hasError = true;
        }

REPLACED WITH

else if ( ! is_email( trim( $_POST['email'] ) ) ) {
                $emailError="You entered an invalid email address.";
                $hasError = true;
            }

You should also look at the wordpress codex on Data Validation, especially the Input Validation on sanitizing user inputs before using them in your code.