This function is EXPERIMENTAL. The behaviour of this function, the name of this function, and anything else documented about this function may change without notice in a future release of PHP. Use this function at your own risk.
mb_regex_encoding() returns the character encoding used by multibyte regex functions.
If the optional parameter encoding is specified, it is set to the character encoding for multibyte regex. The default value is the internal character encoding.
Note: This function is supported in PHP 4.2.0 or higher.