<form name="registerform" id="registerform<?php $template->the_instance(); ?>" action="<?php $template->the_action_url( 'register' ); ?>" method="post">
		<p>
			<label for="user_login<?php $template->the_instance(); ?>"><?php _e( 'Username' ); ?></label>
			<input type="text" name="user_login" id="user_login<?php $template->the_instance(); ?>" class="input" value="<?php $template->the_posted_value( 'user_login' ); ?>" size="20" />
		</p>

		<p>
			<label for="user_email<?php $template->the_instance(); ?>"><?php _e( 'E-mail' ); ?></label>
			<input type="text" name="user_email" id="user_email<?php $template->the_instance(); ?>" class="input" value="<?php $template->the_posted_value( 'user_email' ); ?>" size="20" />
		</p>

		<?php do_action( 'register_form' ); ?>

		<p id="reg_passmail<?php $template->the_instance(); ?>"><?php echo apply_filters( 'tml_register_passmail_template_message', __( 'A password will be e-mailed to you.' ) ); ?></p>

		<p class="submit">
			<input type="submit" name="wp-submit" id="wp-submit<?php $template->the_instance(); ?>" value="<?php esc_attr_e( 'Зарегистрироваться' ); ?>" />
			<input type="hidden" name="redirect_to" value="<?php $template->the_redirect_url( 'register' ); ?>" />
			<input type="hidden" name="instance" value="<?php $template->the_instance(); ?>" />
			<input type="hidden" name="action" value="register" />
		</p>
	</form>

<form>
    <input required="" pattern="[A-Za-z0-9_-]{4,8}" />
    <button type="submit">Зарегаться</button>
</form>

<?php
/*
* Plugin Name: Minimum Password Length
* Description: Enforce a minimum password length.
* Plugin URI: http://www.itsananderson.com/plugins/minimum-password-length
* Plugin Author: Will Anderson
* Author URI: http://www.itsananderson.com/
* Version: 1.0
*/

class WP_Minimum_Password_Length {

public static function start() {
add_action( 'user_profile_update_errors', array( __CLASS__, 'minimum_password_limit' ) );
}

public static function minimum_password_limit( &$errors ) {
// Edit this value to change the minimum password length.
// Set value to zero or disable plugin to remove length requirement.
$min_length = 7;
if ( !empty( $_POST['pass1'] ) && $_POST['pass1'] === $_POST['pass2'] && strlen( $_POST['pass1'] ) < $min_length ) {
$errors->add( 'min_pass_length', sprintf( __( '<strong>ERROR</strong>: Password must be at least %d characters long.' ), $min_length ), array( 'form-field' => 'pass1' ) );
}
}

}

WP_Minimum_Password_Length::start();

<?php
			class WP_Minimum_login_Length {

				public static function start() {
					add_action( 'user_profile_update_errors', array( __CLASS__, 'minimum_login_limit' ) );
}

				public static function minimum_login_limit( &$errors ) {
			$min_length = 4;
		if ( !empty( $_POST['pass1'] ) && $_POST['pass1'] === $_POST['pass2'] && strlen( $_POST['pass1'] ) < $min_length ) {
	$errors->add( 'min_login_length', sprintf( __( '<strong>ERROR</strong>: Password must be at least %d characters long.' ), $min_length ), array( 'form-field' => 'user_login' ) );
}
}

}

WP_Minimum_login_Length::start();

<form action="reg.php" method="post">
   <input name="login" type="text" />
   <input type="submit" value="Send" />
</form>

if(strlen($_POST['login']) < 4) exit('Слишком короткий логин!');

//....code

action="<?php $template->the_action_url( 'register' ); ?>"

<form name="registerform" id="registerform<?php $template->the_instance(); ?>" action="<?php $template->the_action_url( 'register' ); ?>" method="post">

action="reg.php"

if(strlen($_POST['user_login']) < 4) exit('Слишком короткий логин!');

<input type="text" name="user_login" id="user_login<?php $template->the_instance(); ?>" class="input" value="<?php $template->the_posted_value( 'user_login' ); ?>" action="<?php if(strlen($_POST['user_login']) < 4) exit('Слишком короткий логин!'); ?>" size="20" />

if(!preg_match('/^[a-z0-9_-]{4,15}$/i', $_POST['login'])) exit('Неверно заполнено поле «Логин»');

if(!preg_match('/^[a-z0-9_-]{4,15}$/i', $_POST['login'])) exit('Неверно заполнено поле «Логин»');

$key = preg_replace('/[^a-z0-9]/i', '', $key);

$key = preg_replace('/^[a-z0-9_-]{4,15}$/i', '', $key);

function allow_cyrillic_usernames($username, $raw_username, $strict) {
    $username = wp_strip_all_tags( $raw_username );
    $username = remove_accents( $username );
    $username = preg_replace( '|%([a-fA-F0-9][a-fA-F0-9])|', '', $username );
    $username = preg_replace( '/&.+?;/', '', $username );

    if ( $strict )
        $username = preg_replace( '|[^a-zа-я0-9 _.\-@]|iu', '', $username );

    $username = trim( $username );
    $username = preg_replace( '|\s+|', ' ', $username );

    return $username;
}
add_filter('sanitize_user', 'allow_cyrillic_usernames', 10, 3);

if(!preg_match('/^[a-z0-9_-]{4,15}$/i', $key)) exit('Неверно заполнено поле «Логин»');
$key = preg_replace('/[^a-z0-9]/i', '', $key);

if(!preg_match('/^[a-z0-9_-]{4,15}$/i', $key)) exit('Неверно заполнено поле «Логин»');
$key = preg_replace('/[^a-z0-9]/i', '', $key);

/**
 * Handles registering a new user.
 *
 * @param string $user_login User's username for logging in
 * @param string $user_email User's email address to send password and add
 * @return int|WP_Error Either user's ID or error on failure.
 */
function register_new_user( $user_login, $user_email ) {
	$errors = new WP_Error();

	$sanitized_user_login = sanitize_user( $user_login );
	$user_email = apply_filters( 'user_registration_email', $user_email );

	// Check the username
	if ( $sanitized_user_login == '' ) {
		$errors->add( 'empty_username', __( '<strong>ERROR</strong>: Please enter a username.' ) );
	} elseif ( ! validate_username( $user_login ) ) {
		$errors->add( 'invalid_username', __( '<strong>ERROR</strong>: This username is invalid because it uses illegal characters. Please enter a valid username.' ) );
		$sanitized_user_login = '';
		
	} elseif ( username_exists( $sanitized_user_login ) ) {
		$errors->add( 'username_exists', __( '<strong>ERROR</strong>: This username is already registered. Please choose another one.' ) );
	}

	// Check the e-mail address
	if ( $user_email == '' ) {
		$errors->add( 'empty_email', __( '<strong>ERROR</strong>: Please type your e-mail address.' ) );
	} elseif ( ! is_email( $user_email ) ) {
		$errors->add( 'invalid_email', __( '<strong>ERROR</strong>: The email address isn’t correct.' ) );
		$user_email = '';
	} elseif ( email_exists( $user_email ) ) {
		$errors->add( 'email_exists', __( '<strong>ERROR</strong>: This email is already registered, please choose another one.' ) );
	}

	do_action( 'register_post', $sanitized_user_login, $user_email, $errors );

	$errors = apply_filters( 'registration_errors', $errors, $sanitized_user_login, $user_email );

	if ( $errors->get_error_code() )
		return $errors;

	$user_pass = wp_generate_password( 12, false);
	$user_id = wp_create_user( $sanitized_user_login, $user_pass, $user_email );
	if ( ! $user_id ) {
		$errors->add( 'registerfail', sprintf( __( '<strong>ERROR</strong>: Couldn’t register you&hellip; please contact the <a href="mailto:%s">webmaster</a> !' ), get_option( 'admin_email' ) ) );
		return $errors;
	}

	update_user_option( $user_id, 'default_password_nag', true, true ); //Set up the Password change nag.

	wp_new_user_notification( $user_id, $user_pass );

	return $user_id;
}

if(!preg_match('/^[a-z0-9_-]{4,15}$/i', $key)) exit('Неверно заполнено поле «Логин»');