php中preg正则函数使用

1.preg_match和preg_match_all的区别
preg_match和 preg_match_all区别是preg_match只匹配一次。而preg_match_all全部匹配，直到字符串结束。例：

<?php
//注：正则 /a.+?e/ 是非贪婪模式(因为量词‘+’后面加上了‘?’)，如果使用 /a.+?e/U 则变回了贪婪模式
preg_match("/a.+?e/","abcdefgabcdefgabcdefg",$out1);
preg_match_all("/a.+?e/","abcdefgabcdefgabcdefg",$out2);
var_dump($out1);
var_dump($out2);
/*
输出：
array (size=1)
  0 => string 'abcde' (length=5)

array (size=1)
  0 =>
    array (size=3)
      0 => string 'abcde' (length=5)
      1 => string 'abcde' (length=5)
      2 => string 'abcde' (length=5)
 */
?>

2.贪婪模式和非贪婪模式的区别
如：String str="abcaxc";
　　Patter p="ab*c";
贪婪匹配:正则表达式一般趋向于最大长度匹配，也就是所谓的贪婪匹配。如上面使用模式p匹配字符串str，结果就是匹配到：abcaxc(ab*c)。
非贪婪匹配：就是匹配到结果就好，较少的匹配字符。如上面使用模式p匹配字符串str，结果就是匹配到：abc(ab*c)。
例：

<?php
$str = "http://www.baidu/.com?url=www.sina.com/";
preg_match("/http:(.*)com/", $str, $matches1); //贪婪匹配
var_dump($matches1);

preg_match("/http:(.*?)com/", $str, $matches2); //非贪婪匹配(量词'*'后面跟上了'?')
var_dump($matches2);

/*
array (size=2)
  0 => string 'http://www.baidu/.com?url=www.sina.com' (length=38)
  1 => string '//www.baidu/.com?url=www.sina.' (length=30)

array (size=2)
  0 => string 'http://www.baidu/.com' (length=21)
  1 => string '//www.baidu/.' (length=13)
 */
?>

3.preg_match_all参数PREG_PATTERN_ORDER(默认)和PREG_SET_ORDER的区别

<?php
echo('PREG_PATTERN_ORDER');
preg_match_all("|<[^>]+>(.*)</[^>]+>|U",
    "<b>start: </b><b>this is a test</b><b>end</b>",
    $out1);
var_dump($out1);

echo('PREG_SET_ORDER');
preg_match_all("|<[^>]+>(.*)</[^>]+>|U",
    "<b>start: </b><b>this is a test</b><b>end</b>",
    $out2, PREG_SET_ORDER);
var_dump($out2);

/*
PREG_PATTERN_ORDER
array (size=2)
  0 =>
    array (size=3)
      0 => string '<b>start: </b>' (length=14)
      1 => string '<b>this is a test</b>' (length=21)
      2 => string '<b>end</b>' (length=10)
  1 =>
    array (size=3)
      0 => string 'start: ' (length=7)
      1 => string 'this is a test' (length=14)
      2 => string 'end' (length=3)

PREG_SET_ORDER
array (size=3)
  0 =>
    array (size=2)
      0 => string '<b>start: </b>' (length=14)
      1 => string 'start: ' (length=7)
  1 =>
    array (size=2)
      0 => string '<b>this is a test</b>' (length=21)
      1 => string 'this is a test' (length=14)
  2 =>
    array (size=2)
      0 => string '<b>end</b>' (length=10)
      1 => string 'end' (length=3)
 */
?>

延伸阅读：preg_match_all使用实例