Source: Internet
Author: User
Tags preg expression engine
: This article mainly introduces the php-preg-1, for PHP tutorials interested in students can refer. Q. Regular expression

Regular expressions are composed of three parts: atoms, metacharacters, and pattern correction. Atoms are common characters.


\ D: match any decimal number [0-9]

\ S: matches any blank character

\ W: match any number, letter, or underline [0-9a-zA-Z _]

.: Match any character except the line break

? : Match 0 times or once

+: Match the previous atom once or multiple times

*: Match any time

|: Match two or more branch selections to separate multiple select modes

\ B: match the word boundary

\ B: non-word boundary

^: Match the start position of the input string

$: End position of matching input string

(): A Pattern unit. multiple atoms form a large atom as a unit. In this way, the matching information is stored in a cache and can be obtained for future use. The buffer number for storing sub-matches starts from 1 and ranges from 99 to 99. Each buffer can be accessed using \ n. Note the escape when using the regular expression mode. For example, "/^ \ d {4} (\ W) \ d {2} \ 1 \ d {2} $ /". Can be used in brackets? : S: Set the non-storage mode.

Internal working mechanism of the regular expression engine

Knowing how the regular expression engine works helps you quickly understand why a regular expression does not work as expected.

There are two types of engines: the text-directed engine and the regex-directed engine. Jeffrey Friedl calls them the DFA and NFA engines. This article talks about the regular expression-oriented engine. This is because some very useful features, such as lazy quantifiers and backreferences, can only be implemented in the regular expression-oriented engine. So it is not surprising that this engine is currently the most popular.

You can easily tell whether the engine is text-oriented or regular-expression-oriented. If reverse references or "inert" quantifiers are implemented, you are sure that the engine you are using is regular-oriented. You can test the regular expression as follows: <

The above introduces the php-preg-1, including the content of the aspect, hope to be interested in PHP Tutorial friends help.

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