下列常量由此扩展定义，且仅在此扩展编译入 PHP 或在运行时动态载入时可用。
| constant | description |
|---|---|
| PREG_PATTERN_ORDER | Orders results so that $matches[0] is an array of full pattern matches, $matches[1] is an array of strings matched by the first parenthesized subpattern, and so on. This flag is only used with preg_match_all(). |
| PREG_SET_ORDER | Orders results so that $matches[0] is an array of first set of matches, $matches[1] is an array of second set of matches, and so on. This flag is only used with preg_match_all(). |
| PREG_OFFSET_CAPTURE | See the description of PREG_SPLIT_OFFSET_CAPTURE. This flag is available since PHP 4.3.0. |
| PREG_SPLIT_NO_EMPTY | This flag tells preg_split() to return only non-empty pieces. |
| PREG_SPLIT_DELIM_CAPTURE | This flag tells preg_split() to capture parenthesized expression in the delimiter pattern as well. This flag is available since PHP 4.0.5. |
| PREG_SPLIT_OFFSET_CAPTURE | If this flag is set, for every occurring match the appendant string offset will also be returned. Note that this changes the return values in an array where every element is an array consisting of the matched string at offset 0 and its string offset within subject at offset 1. This flag is available since PHP 4.3.0 and is only used for preg_split(). |
| PREG_NO_ERROR | Returned by preg_last_error() if there were no errors. Available since PHP 5.2.0. |
| PREG_INTERNAL_ERROR | Returned by preg_last_error() if there was an internal PCRE error. Available since PHP 5.2.0. |
| PREG_BACKTRACK_LIMIT_ERROR | Returned by preg_last_error() if backtrack limit was exhausted. Available since PHP 5.2.0. |
| PREG_RECURSION_LIMIT_ERROR | Returned by preg_last_error() if recursion limit was exhausted. Available since PHP 5.2.0. |
| PREG_BAD_UTF8_ERROR | Returned by preg_last_error() if the last error was caused by malformed UTF-8 data (only when running a regex in UTF-8 mode). Available since PHP 5.2.0. |
| PREG_BAD_UTF8_OFFSET_ERROR | Returned by preg_last_error() if the offset didn't correspond to the begin of a valid UTF-8 code point (only when running a regex in UTF-8 mode). Available since PHP 5.3.0. |
| PCRE_VERSION | PCRE version and release date (e.g. "7.0 18-Dec-2006"). Available since PHP 5.2.4. |