Beginning with PHP 4.2.0 these functions are enabled by default. You can disable the pcre functions with --without-pcre-regex. Use --with-pcre-regex=DIR to specify DIR where PCRE's include and library files are located, if not using bundled library. For older versions you have to configure and compile PHP with --with-pcre-regex[=DIR] in order to use these functions.
The Windows version of PHP has built-in support for this extension. You do not need to load any additional extensions in order to use these functions.
Note: As of PHP 5.3.0 this extension cannot be disabled and is therefore always present.
It is still possible to build against an external PCRE library by using --with-pcre-regex=DIR