The syntax for patterns used in these functions closely resembles Perl. The expression should be enclosed in the delimiters, a forward slash (/), for example. Any character can be used for delimiter as long as it's not alphanumeric or backslash (\). If the delimiter character has to be used in the expression itself, it needs to be escaped by backslash. Since PHP 4.0.4, you can also use Perl-style (), {}, [], and <> matching delimiters.
The ending delimiter may be followed by various modifiers that affect the matching. See Pattern Modifiers.
PHP also supports regular expressions using a POSIX-extended syntax using the POSIX-extended regex functions..
Regular expression support is provided by the PCRE library package, which is open source software, written by Philip Hazel, and copyright by the University of Cambridge, England. It is available at ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/.
Beginning with PHP 4.2.0 these functions are enabled by default. You can disable the pcre functions with --without-pcre-regex. 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 extension in order to use these functions.
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.
Table 1. 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 only 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 occuring match the appendant string offset will also be returned. Note that this changes the return value in an array where very element is an array consisting of the matched string at offset 0 and it's string offset into subject at offset 1. This flag is available since PHP 4.3.0 and is only used for preg_split(). |