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.
http://weitz.de/files/regex-coach.exe (Windows installer) http://weitz.de/files/regex-coach.tgz (Linux tar archive)
PCRE 関数
目次
- preg_filter — 正規表現による検索と置換を行う
- preg_grep — パターンにマッチする配列の要素を返す
- preg_last_error_msg — 最後に実行した PCRE 正規表現に関するエラーメッセージを返す
- preg_last_error — 直近の PCRE 正規表現処理のエラーコードを返す
- preg_match_all — 繰り返し正規表現検索を行う
- preg_match — 正規表現によるマッチングを行う
- preg_quote — 正規表現文字をクオートする
- preg_replace_callback_array — 正規表現検索を行い、コールバック関数を使用して置換を行う
- preg_replace_callback — 正規表現検索を行い、コールバック関数を使用して置換を行う
- preg_replace — 正規表現検索および置換を行う
- preg_split — 正規表現で文字列を分割する
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User Contributed Notes 4 notes
steve at stevedix dot de ¶
20 years ago
theking2 at king dot ma ¶
1 year ago
Writing regexp can be tedious and error prone. Test yours on https://regex101.com/ to make sure you haven't missed anything . It even colour codes example strings.
stronk7 at moodle dot org ¶
17 years ago
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 :-)
Svoop ¶
15 years ago
I have written a short introduction and a colorful cheat sheet for Perl Compatible Regular Expressions (PCRE):
http://www.bitcetera.com/en/techblog/2008/04/01/regex-in-a-nutshell/
