Php-preg-1

Q. Regular Expressions

Regular expressions are made up of atoms, meta-characters, and pattern corrections in three parts. The atom is also the ordinary character.

Metacharacters

\d: Matches any decimal number [0-9]

\s: Matches any white space character

\w: Matches any number, letter, underscore [0-9a-za-z_]

.: Matches any of the characters except the newline character alien

?: Match 0 or one time

+: Match 1 or more times before the atom

*: Matches any time

|: Match two or more branch selections to separate the mode of multiple selection

\b: Match the boundaries of a word

\b: Non-word boundary

^: match the starting position of the input string

$: Match the end position of the input string

(): A mode unit that makes a large atom of multiple atoms, as a unit. This stores the relevant match in a buffer and can be retrieved for later use. The buffer number for the storage sub-match starts at 1, up to 99, and each buffer can be accessed using \ n. Be aware of escaping when used in regular expression patterns. For example: "/^\d{4} (\w) \d{2}\1\d{2}$/". Can be used in parentheses?: S to set non-storage mode.

The internal working mechanism of the regular expression engine

Knowing how the regular expression engine works helps you quickly understand why a regular expression doesn't work as you expect.

  有两种类型的引擎：文本导向(text-directed)的引擎和正则导向(regex-directed)的引擎。Jeffrey Friedl把他们称作DFA和NFA引擎。本文谈到的是正则导向的引擎。这是因为一些非常有用的特性，如“惰性”量词(lazy quantifiers)和反向引用(backreferences)，只能在正则导向的引擎中实现。所以毫不意外这种引擎是目前最流行的引擎。

You can easily tell if the engine you are using is text-oriented or regular-oriented. If a reverse reference or "lazy" quantifier is implemented, you can be sure that the engine you are using is regular-oriented. You can test the following: Regular Expressions <

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