Pcre php что это
The syntax and semantics of the regular expressions supported by PCRE are described below. Regular expressions are also described in the Perl documentation and in a number of other books, some of which have copious examples. Jeffrey Friedl’s «Mastering Regular Expressions», published by O’Reilly (ISBN 1-56592-257-3), covers them in great detail. The description here is intended as reference documentation.
A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for themselves in a pattern, and match the corresponding characters in the subject. As a trivial example, the pattern The quick brown fox matches a portion of a subject string that is identical to itself.
User Contributed Notes 1 note
PCRE is an acronym for Perl Compatible Regular Expressions. «The PCRE library is a set of functions that implement regular expression pattern matching using the same syntax and semantics as Perl 5.» (from: pcre dot org)
- PCRE regex syntax
- Introduction
- Delimiters
- Meta-characters
- Escape sequences
- Unicode character properties
- Anchors
- Dot
- Character classes
- Alternation
- Internal option setting
- Subpatterns
- Repetition
- Back references
- Assertions
- Once-only subpatterns
- Conditional subpatterns
- Comments
- Recursive patterns
- Performance
Функции PCRE
Something to bear in mind is that regex is actually a declarative programming language like prolog : your regex is a set of rules which the regex interpreter tries to match against a string. During this matching, the interpreter will assume certain things, and continue assuming them until it comes up against a failure to match, which then causes it to backtrack. Regex assumes «greedy matching» unless explicitly told not to, which can cause a lot of backtracking. A general rule of thumb is that the more backtracking, the slower the matching process.
It is therefore vital, if you are trying to optimise your program to run quickly (and if you can’t do without regex), to optimise your regexes to match quickly.
I recommend the use of a tool such as «The Regex Coach» to debug your regex strings.
One comment about 5.2.x and the pcre.backtrack_limit:
Note that this setting wasn’t present under previous PHP releases and the behaviour (or limit) under those releases was, in practise, higher so all these PCRE functions were able to «capture» longer strings.
With the arrival of the setting, defaulting to 100000 (less than 100K), you won’t be able to match/capture strings over that size using, for example «ungreedy» modifiers.
So, in a lot of situations, you’ll need to raise that (very small IMO) limit.
The worst part is that PHP simply won’t match/capture those strings over pcre.backtrack_limit and will it be 100% silent about that (I think that throwing some NOTICE/WARNING if raised could help a lot to developers).
There is a lot of people suffering this changed behaviour from I’ve read on forums, bugs and so on).
Hope this note helps, ciao 🙂
Синтаксис регулярных выражений
Warning: preg_match() [function.preg-match]: Compilation failed: PCRE does not support \L, \l, \N, \P, \p, \U, \u, or \X at .
As this manual page says, you need PHP 5.1.0 and the /u modifier in order to enable these features, but that isn’t the only requirement! It is possible to install later versions of PHP (we have 5.1.4) while linking to an older PCRE install. A quick look at the PCRE changelog suggests that you probably need at least PCRE 5; we’re running 4.5, while the latest is 7.1. You can find out your PCRE version by checking phpinfo().
I suspect this ancient PCRE version is included in some officially-supported Red Hat Enterprise package which is probably why we are running it so might also affect other people.
In the character class meta-character documentation above, the circumflex (^) is described:
«^ negate the class, but only if the first character»
It should be a little more verbose to fully express the meaning of ^:
^ Negate the character class. If used, this must be the first character of the class (e.g. «[^012]»).
Pay attention that some pcre features such as once-only or recursive patterns are not implemented in php versions prior to 5.00
ive never used regex expressions till now and had loads of difficulty trying to convert a [url]link here[/url] into an href for use with posting messages on a forum, heres what i manage to come up with:
$patterns = array(
«/\[link\](.*?)\[\/link\]/»,
«/\[url\](.*?)\[\/url\]/»,
«/\[img\](.*?)\[\/img\]/»,
«/\[b\](.*?)\[\/b\]/»,
«/\[u\](.*?)\[\/u\]/»,
«/\[i\](.*?)\[\/i\]/»
);
$replacements = array(
«\\1»,
«\\1»,
«»,
«\\1«,
«\\1«,
«\\1«);
$newText = preg_replace($patterns,$replacements, $text);at first it would collect ALL the tags into one link/bold/whatever, until i added the «?» i still dont fully understand it. but it works 🙂
About strip_selected_tags function from two posts below:
it does not work if somebody uses tags without ending «>» character, like this:
This is even valid HTML (but not valid XHTML)
As a rule of thumb, it’s better to describe your regular expression patterns using single-quoted strings.
Using double-quoted strings, the interaction between PHP’s and PCRE’s interpretations of which bits of the string are escape sequences can get messy. Regular expressions can get messy enough as it is without another layer of escaping making it worse.
Concerning note #6 in «Differences From Perl», the \G token *is* supported as the last match position anchor. This has been confirmed to work at least in preg_replace(), though I’d assume it’d work in preg_match_all(), and other functions that can make more than one match, as well.
If, like me, you tend to use the /U pattern modifier, then you will need to remember that using ? or * to to test for optional characters will match zero characters if it means that the rest of the pattern can continue matching, even if the optional characters exist.
For instance, if we have this string:
The whole pattern is matched but none of the _ characters are placed in the sub-pattern. The way around this (if you still wish to use /U) is to use the ? greediness inverter. eg,