leaky-paywall-limit-level-id-signup.php

<?php

// this requires an edu email address to sign up for the level.  Can be changed to whatever the requirements are.

// front end check
add_filter('leaky_paywall_account_setup_validation', function ($errors, $fields) {

	$edu_level_id = '6';

	if (isset($fields['level_id']) && $edu_level_id === $fields['level_id']) {
		$email = isset($fields['email_address']) ? $fields['email_address'] : '';
		$domain = substr(strrchr($email, '@'), 1);

		if (! $domain || substr(strtolower($domain), -4) !== '.edu') {
			$errors['edu_email'] = array(
				'message' => __('This subscription level requires a .edu email address.', 'leaky-paywall'),
			);
		}
	}
	return $errors;
}, 10, 2);


// server side check for redundancy 
add_filter( 'leaky_paywall_registration_form_errors', function( $errors, $fields ) {

	$edu_level_id = '5';

    if ( isset( $fields['level_id'] ) && $edu_level_id === $fields['level_id'] ) {
        $email = isset( $fields['email'] ) ? $fields['email'] : '';
        $domain = substr( strrchr( $email, '@' ), 1 );

        if ( ! $domain || substr( strtolower( $domain ), -4 ) !== '.edu' ) {
            $errors[] = 'This subscription level requires a .edu email address.';
        }
    }
    return $errors;
}, 10, 2 );