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PHP capturing groups and non-capturing groups

墨辰丷
墨辰丷Original
2018-06-06 11:01:061294browse

This article mainly introduces the relevant information of capturing groups and non-capturing groups in PHP regular expressions. Friends who need it can refer to it

I encountered a regular matching problem today, and suddenly I found that there is capture. The concept of groups is also ignored in the manual. When I turned to Baidu, I accidentally found that there are special uses of regular capture groups in C# and Java. When I searched for the keyword PHP, there was no relevant content. I tried it myself and found that in PHP It is also feasible, so I will summarize it. While sharing, I also hope that some experts and careful learners can find the problems in my understanding.

What is a capturing group

Capturing group syntax:

##\Reverse of capturing group Quote. where num is a positive integer. (\w)(\w)\2\1 \kab9b28e019fca8075a5594c7291c0cef or

\k'name'

##

我们先看一下PHP的正则匹配函数

int preg_match ( string $pattern , string $subject [, array &$matches [, int $flags = 0 [, int $offset = 0 ]]] )

前面两项是我们常用的,$pattern是正则匹配模式,$string是要匹配的字符串。

array &$match,它是一个数组,&表示匹配出来的结果会被写入$match中。

int $flags 如果传递了这个标记, 对于每一个出现的匹配返回时会附加字符串偏移量(相对于目标字符串的)。

int $offset 用于指定从目标字符串的某个未知开始搜索(单位是字节)。

我们主要看一下$match的值里会有什么:

$mode = '/a=(\d+)b=(\d+)c=(\d+)/';
$str='**a=4b=98c=56**';
$res=preg_match($mode,$str,$match);
var_dump($match);

结果如下:

array (size=4)
  0 => string 'a=4b=98c=56' (length=11)
  1 => string '4' (length=1)
  2 => string '98' (length=2)
  3 => string '56' (length=2)

现在我们知道了什么是捕获组,捕获组是正则表达示中以()括起来的部分,每一对()是一个捕获组。

PHP会为它编号,从1开始。至于为什么会从1开始,那是因为PHP把匹配到的完整字符串编号为0。

如果有多个括号或嵌套括号,按左边括号出现的顺序来进行编号,如图:

按图中的匹配模式匹配时,捕获组的123号分别是红绿蓝。

捕获组的忽略与命名

我们还可以阻止PHP为匹配组的编号:在匹配组中模式前加  ?:

$mode = '/a=(\d+)b=(?:\d+)c=(\d+)/';

这样,匹配结果就会变成:

array (size=3)
 0 => string 'a=4b=98c=56' (length=11)
 1 => string '4' (length=1)
 2 => string '56' (length=2)

当然,我们也可以在括号的内部为它给它独特的名字。

命名子组可以接受(?8a11bc632ea32a57b3e3693c7987c420), (?'name') 以及(?P8a11bc632ea32a57b3e3693c7987c420)语法. 之前版本仅接受(?P8a11bc632ea32a57b3e3693c7987c420)语法.

例如:$mode = '/a=(\d+)b=(?P6fd21b5631009a6df3e3d5bae31abfd3\d+)c=(\d+)/';

使用时结果为:

array (size=5)
 0 => string 'a=4b=98c=56' (length=11)
 1 => string '4' (length=1)
 'sec' => string '98' (length=2)
 2 => string '98' (length=2)
 3 => string '56' (length=2)

在保留索引数组的同时,加上一个关联项,key值为捕获组名。

捕获组的反向引用

我们在用preg_replace()函数进行正则替换时,我们还可以使用 \n 或 $n 来引用第n个捕获组.

$mode = '/a=(\d+)b=(\d+)c=(\d+)/';
$str='**a=4b=98c=56**';
$rp='\1/$2/\3/';
echo preg_replace($mode,$rp,$str);//**4/98/56/**

\1表示捕获组1(4),$2为捕获组2(98),\3为捕获组3(56)。

非捕获组的用法:

非捕获组语法:

Characters

Description

Example

##(

pattern)

Match pattern and capture As a result, the group number is automatically set.

(abc) d

matches abcd or abcabcd

##(?1e9a2a7d2a1b62d92d5600cb10be2556pattern) or

( ?'

name'pattern)

Match pattern and capture the result, set name to the group name.

num

matches abba

Backreference to the named capture group. where name is the capture group name.

(?1cd70caa06cc6f4a046ab31342a5e7b4\w)abc\k1cd70caa06cc6f4a046ab31342a5e7b4

Match xabcx

字符 

描述

Example

##(?:

pattern)

Match pattern, but do not capture the matching result.

'industr(?:y|ies)

Matches 'industry' or 'industries'.

(?=

pattern)

Zero-width forward lookup, no capture Matching results.

'Windows (?=95|98|NT|2000)'

Matches "Windows" in "Windows2000"

Does not match" "Windows" in Windows3.1".

(?!pattern)

Zero-width negative lookup, does not capture matching results.

'Windows (?!95|98|NT|2000)'

Matches "Windows" in "Windows3.1"

No Matches "Windows" in "Windows2000".

(?<=pattern)

Zero width forward lookback, Matches are not captured.

'2000 (?<=Office|Word|Excel)'

matches "2000" in "Office2000"

does not match" "2000" in Windows2000".

(?pattern)

零宽度负向回查,不捕获匹配结果。

'2000 (?

匹配 " Windows2000" 中的 "2000"

不匹配 " Office2000" 中的 "2000"。

为什么称为非捕获组呢?那是因为它们有捕获组的特性,在匹配模式的()中,但是匹配时,PHP不会为它们编组,它们只会影响匹配结果,并不作为结果输出。

/d(?=xxx)    匹配"后面是xxx的一个数字"。

注意格式:只能放在匹配模式字符串之后!

例如:

$pattern=&#39;/\d(?=abc)/&#39;;
$str="ab36abc8eg";
$res=preg_match($pattern,$str,$match);
var_dump($match);//6

匹配的6,因为只有它作为一个数字,后面还有abc。

(?<=xxx) /d 匹配"前面是xxx的一个数字"

注意格式:只能放在匹配模式字符串之前!

例如:

$pattern=&#39;/(?<=abc)\d/&#39;;
$str="ab36abc8eg";
$res=preg_match($pattern,$str,$match);
var_dump($match);//8

匹配的8,因为只有它作为一个数字,后面还有abc。

与(?=xxx)  (?<=xxx)相对的是(?!=xxx)  (?

它表示前面/后面不是xxx的字符串,这里就不再举例了。

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