Setup
The POSIX regular expression extension is enabled in PHP by default, unless explicitly disabled by specifying the --disable-regex switch to the configuration program before compiling it. The one trick to this is that the extension only supports multi-byte strings if you also enable the mbstring extension. Microsoft Windows users merely need to make sure the following line in php.ini has no semicolon ( ; ) character in front of it:
extension=php_mbstring.dll
You have no configuration options for this extension in php.ini, so after it is compiled or enabled, it is ready for use.