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PCRE
PHP Manual

Predefined Constants

The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.

PREG constants
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.

PCRE
PHP Manual