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Analysis of capturing groups and non-capturing groups in PHP regular expressions

Jun 01, 2018 pm 05:30 PM
php Case Analysis capture

本篇文章主要介绍PHP正则中的捕获组与非捕获组实例分析,感兴趣的朋友参考下,希望对大家有所帮助。

什么是捕获组

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 $offset = 0 ]]] )
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前面两项是我们常用的,$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);
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结果如下:

array (size=4)
=> string 'a=4b=98c=56' (length=11)
=> string '4' (length=1)
=> string '98' (length=2)
=> string '56' (length=2)
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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)
=> string 'a=4b=98c=56' (length=11)
=> string '4' (length=1)
=> string '56' (length=2)
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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(?), (?'name') 以及(?P)语法. 之前版本仅接受(?P)语法.

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

使用时结果为:

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

 'sec' => string '98' (length=2)
=> string '98' (length=2)
=> string '56' (length=2)
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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 $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/**
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\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:

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='/\d(?=abc)/';

$str="ab36abc8eg";

$res=preg_match($pattern,$str,$match);

var_dump($match);//6
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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
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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。

总结:以上就是本篇文的全部内容,希望能对大家的学习有所帮助。

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