'recode' WordPress function - is_email()

readme.md

if ( is_email( '[email protected]' ) ) {
    echo 'email address is valid.';
}

functions.php

<?php
/**
 * Verifies that an email is valid.
 *
 * @param $email (string) (Required) Email address to verify.
 * @return (string|bool) Either false or the valid email address.
 */
function is_email( $email ){
    // Test for the minimum length the email can be
	if ( mb_strlen( $email ) < 6 ) {
		/**
		 * Filters whether an email address is valid.
		 *
		 * This filter is evaluated under several different contexts, such as 'email_too_short',
		 * 'email_no_at', 'local_invalid_chars', 'domain_period_sequence', 'domain_period_limits',
		 * 'domain_no_periods', 'sub_hyphen_limits', 'sub_invalid_chars', or no specific context.
		 *
		 * @since 2.8.0
		 *
		 * @param bool   $is_email Whether the email address has passed the is_email() checks. Default false.
		 * @param string $email    The email address being checked.
		 * @param string $context  Context under which the email was tested.
		 */
		return apply_filters( 'is_email', false, $email, 'email_too_short' );
	}

	if( function_exists( 'filter_var' ) ){
		$is_valid = (bool) filter_var( $email, FILTER_VALIDATE_EMAIL );

		$context = $email;

		if( ! $is_valid ){
			$context = false;
		}

		return apply_filters( 'is_email', $context, $email, null );
	}
	else{
		// Test for an @ character after the first position
		if ( strpos( $email, '@', 1 ) === false ) {
      /** This filter is documented in wp-includes/formatting.php */
			return apply_filters( 'is_email', false, $email, 'email_no_at' );
		}
		
		// Split out the local and domain parts
		list( $local, $domain ) = explode( '@', $email, 2 );
		
		// LOCAL PART
		// Test for invalid characters
		if ( ! preg_match( '/^[a-zA-Z0-9!#$%&\'*+\/=?^_`{|}~\.-]+$/', $local ) ) {
      /** This filter is documented in wp-includes/formatting.php */
			return apply_filters( 'is_email', false, $email, 'local_invalid_chars' );
		}
		
		// DOMAIN PART
		// Test for sequences of periods
		if ( preg_match( '/\.{2,}/', $domain ) ) {
      /** This filter is documented in wp-includes/formatting.php */
			return apply_filters( 'is_email', false, $email, 'domain_period_sequence' );
		}
		
		// Test for leading and trailing periods and whitespace
		if ( trim( $domain, " \t\n\r\0\x0B." ) !== $domain ) {
      /** This filter is documented in wp-includes/formatting.php */
			return apply_filters( 'is_email', false, $email, 'domain_period_limits' );
		}
		
		// Split the domain into subs
		$subs = explode( '.', $domain );
		
		// Assume the domain will have at least two subs
		if ( 2 > count( $subs ) ) {
      /** This filter is documented in wp-includes/formatting.php */
			return apply_filters( 'is_email', false, $email, 'domain_no_periods' );
		}
		
		// Loop through each sub
		foreach ( $subs as $sub ) {
			// Test for leading and trailing hyphens and whitespace
			if ( trim( $sub, " \t\n\r\0\x0B-" ) !== $sub ) {
        /** This filter is documented in wp-includes/formatting.php */
				return apply_filters( 'is_email', false, $email, 'sub_hyphen_limits' );
			}
		
			// Test for invalid characters
			if ( ! preg_match( '/^[a-z0-9-]+$/i', $sub ) ) {
        /** This filter is documented in wp-includes/formatting.php */
				return apply_filters( 'is_email', false, $email, 'sub_invalid_chars' );
			}
		}
		
		// Congratulations your email made it!
		/** This filter is documented in wp-includes/formatting.php */
		return apply_filters( 'is_email', $email, $email, null );
	}
}