PHP正则 贪婪模式与懒惰模式
贪婪模式
当正则表达式中包含能接受重复的限定符时,通常的行为是(在使整个表达式能得到匹配的前提下)匹配尽可能多的字符。
考虑这个表达式:a.*b,它将会匹配最长的以a开始,以b结束的字符串。如果用它来搜索aabab的话,它会匹配整个字符串aabab。这被称为贪婪匹配。
懒惰模式
有时,我们更需要懒惰匹配,也就是匹配尽可能少的字符。前面给出的限定符都可以被转化为懒惰匹配模式,只要在它后面加上一个问号?。这样.*?就意味着匹配任意数量的重复,但是在能使整个匹配成功的前提下使用最少的重复。现在看看懒惰版的例子吧:
a.*?b匹配最短的,以a开始,以b结束的字符串。如果把它应用于aabab的话,它会匹配aab(第一到第三个字符)和ab(第四到第五个字符)。
例子2
要求字符串: src="http://www.bloghome.cn/1.mp3" type="application/x-mplayer2" 中找到匹配结果:http://www.bloghome.cn/1.mp3
如果匹配表达式写为:/src="(.*)"/,
$str ='src="http://www.bloghome.cn/1.mp3" type="application/x-mplayer2"'; preg_match('/src="(.*)"/', $str, $matches);
得到$matches[1]结果为http://www.bloghome.cn/1.mp3" type="application/x-mplayer2因为最后一个双引号的匹配是贪婪模式的。
如果匹配表达式写为:/src="(.*?)"/,得到$matches[1]结果为http://www.bloghome.cn/1.mp3
结尾定界符/后面的含义
注意, 若pattern中使用了
/s 表示.还匹配空白字符
/U 表示模式反转, 即贪婪匹配模式变成懒惰匹配模式, 懒惰匹配模式变成贪婪匹配模式

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