is_email()

is_email( string $email, bool $deprecated = false ): string|false

function is_email( $email, $deprecated = false ) {
	if ( ! empty( $deprecated ) ) {
		_deprecated_argument( __FUNCTION__, '3.0.0' );
	}

	// Test for the minimum length the email can be.
	if ( 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 string|false $is_email The email address if successfully passed the is_email() checks, false otherwise.
		 * @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' );
	}

	// Test for an @ character after the first position.
	if ( false === strpos( $email, '@', 1 ) ) {
		/** 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 );
}