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_mb_strlen() WP 4.2.0

Internal compat function to mimic mb_strlen().

Only understands UTF-8 and 8bit. All other character sets will be treated as 8bit. For $encoding === UTF-8, the $str input is expected to be a valid UTF-8 byte sequence. The behavior of this function for invalid inputs is undefined.

This is an internal function for using it by WP core itself. It's not recommended to use this function in your code.

No Hooks.

Return

Int. String length of $str.

Usage

_mb_strlen( $str, $encoding );
$str(string) (required)
The string to retrieve the character length from.
$encoding(string|null)
Character encoding to use.
Default: null

Changelog

Since 4.2.0 Introduced.

Code of _mb_strlen() WP 5.7.2

<?php
function _mb_strlen( $str, $encoding = null ) {
	if ( null === $encoding ) {
		$encoding = get_option( 'blog_charset' );
	}

	/*
	 * The solution below works only for UTF-8, so in case of a different charset
	 * just use built-in strlen().
	 */
	if ( ! in_array( $encoding, array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ), true ) ) {
		return strlen( $str );
	}

	if ( _wp_can_use_pcre_u() ) {
		// Use the regex unicode support to separate the UTF-8 characters into an array.
		preg_match_all( '/./us', $str, $match );
		return count( $match[0] );
	}

	$regex = '/(?:
		[\x00-\x7F]                  # single-byte sequences   0xxxxxxx
		| [\xC2-\xDF][\x80-\xBF]       # double-byte sequences   110xxxxx 10xxxxxx
		| \xE0[\xA0-\xBF][\x80-\xBF]   # triple-byte sequences   1110xxxx 10xxxxxx * 2
		| [\xE1-\xEC][\x80-\xBF]{2}
		| \xED[\x80-\x9F][\x80-\xBF]
		| [\xEE-\xEF][\x80-\xBF]{2}
		| \xF0[\x90-\xBF][\x80-\xBF]{2} # four-byte sequences   11110xxx 10xxxxxx * 3
		| [\xF1-\xF3][\x80-\xBF]{3}
		| \xF4[\x80-\x8F][\x80-\xBF]{2}
	)/x';

	// Start at 1 instead of 0 since the first thing we do is decrement.
	$count = 1;
	do {
		// We had some string left over from the last round, but we counted it in that last round.
		$count--;

		/*
		 * Split by UTF-8 character, limit to 1000 characters (last array element will contain
		 * the rest of the string).
		 */
		$pieces = preg_split( $regex, $str, 1000 );

		// Increment.
		$count += count( $pieces );

		// If there's anything left over, repeat the loop.
	} while ( $str = array_pop( $pieces ) );

	// Fencepost: preg_split() always returns one extra item in the array.
	return --$count;
}