# 正则表达式

正则表达式的作用：分割、查找、匹配、替换字符串

\w 是数字字母下划线，并不是只包含字母

## 后向引用

```php
<?php
$str = '<b>abc</b>';
$pattern = '/<b>(.*)<\/b>/';
$str = preg_replace($pattern, '\\1', $str);
var_dump($str);
```

## 贪婪模式

```php
<?php
$str = '<b>abc</b><b>bcd</b>';
$pattern = '/<b>.*?<\/b>/';
//$pattern = '/<b>.*?<\/b>/U';
$strs = preg_match_all($pattern, $str, $match);
var_dump($match);
```

## 正则表达式PCRE函数：

* preg\_match
* preg\_match\_all()、
* preg\_replace()
* preg\_split()

## 中文匹配

UTF-8汉字编码范围是0x4e00-0x9fa5,在ANSI(gb2312)环境,0xb0-0xf7,0xa1-0xfe

UTF-8要使用u模式修正符使模式字符串被当成UTF-8，在ANSI(gb2312)环境下,要使用chr将Ascii码转换为字符

```php
<?php

$str = '中文';

$pattern = '/[\x{4e00}-\x{9fa5}]/u';

//$pattern = '/['.chr(0xb0).'-'.chr(0xf7).']['.chr(0xa1).'-'.chr(0xfe).']/';

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

var_dump($match);
```

## 示例：

```php
<?php

// 请写出以139开头的11位手机号码的正则表达式

// 13988888888
//
$str = '13888888888';

$pattern = '/^139\d{8}$/';

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

var_dump($match);
```

```php
<?php

// 请匹配所有img标签中的src的值

$str = '<img alt="高清无码" id="av" src="av.jpg" />';

$pattern = '/<img.*?src="(.*?)".*?\/?>/i';

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

var_dump($match);
```


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