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.
See Pattern Syntax
for detailed explanation.
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.
Note:
This extension maintains a global per-thread cache of compiled regular
expressions (up to 4096).
No external libraries are needed to build this extension.
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
extension in order to use these functions.
The behaviour of these functions is affected by settings in php.ini.
Table 1. PCRE Configuration Options
| Name | Default | Changeable | Changelog |
|---|
| pcre.backtrack_limit | 100000 | PHP_INI_ALL | Available since PHP 5.2.0. |
| pcre.recursion_limit | 100000 | PHP_INI_ALL | Available since PHP 5.2.0. |
For further details and definitions of the
PHP_INI_* constants, see the
Appendix G.
Here's a short explanation of
the configuration directives.
- pcre.backtrack_limit
integer
PCRE's backtracking limit.
- pcre.recursion_limit
integer
PCRE's recursion limit. Please note that if you set this value to a high
number you may consume all the available process stack and eventually
crash PHP (due to reaching the stack size limit imposed by the Operating
System).
This extension has no resource types defined.
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 2. 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, $matches[1] is an array of second set of matches, and so
on. This flag is only used with preg_match_all().
|
| PREG_OFFSET_CAPTURE |
See the description of
PREG_SPLIT_OFFSET_CAPTURE. This flag is
available since PHP 4.3.0.
|
| PREG_SPLIT_NO_EMPTY |
This flag tells preg_split() to return only non-empty
pieces.
|
| PREG_SPLIT_DELIM_CAPTURE |
This flag tells preg_split() to capture
parenthesized expression in the delimiter pattern as well. This flag
is available since PHP 4.0.5.
|
| PREG_SPLIT_OFFSET_CAPTURE |
If this flag is set, for every occurring match the appendant string
offset will also be returned. Note that this changes the return
values in an array where every element is an array consisting of the
matched string at offset 0 and its string offset within subject at
offset 1. This flag is available since PHP 4.3.0
and is only used for preg_split().
|
| PREG_NO_ERROR |
Returned by preg_last_error() if there were no
errors. Available since PHP 5.2.0.
|
| PREG_INTERNAL_ERROR |
Returned by preg_last_error() if there was an
internal PCRE error. Available since PHP 5.2.0.
|
| PREG_BACKTRACK_LIMIT_ERROR |
Returned by preg_last_error() if backtrack limit was exhausted.
Available since PHP 5.2.0.
|
| PREG_RECURSION_LIMIT_ERROR |
Returned by preg_last_error() if recursion limit was exhausted.
Available since PHP 5.2.0.
|
| PREG_BAD_UTF8_ERROR |
Returned by preg_last_error() if the last error was
caused by malformed UTF-8 data (only when running a regex in UTF-8 mode). Available
since PHP 5.2.0.
|
Example 1. Examples of valid patterns /<\/\w+>/ |(\d{3})-\d+|Sm /^(?i)php[34]/ {^\s+(\s+)?$}
|
Example 2. Examples of invalid patterns
/href='(.*)' - missing ending delimiter
/\w+\s*\w+/J - unknown modifier 'J'
1-\d3-\d3-\d4| - missing starting delimiter
|
pcntl_wtermsig
