关于php正则表达式的两点备注

several tips about Regular Expressions
1. process for "greedy"
     By default, the quantifiers  are  "greedy",  that  is,  they
     match  as much as possible (up to the maximum number of per-
     mitted times), without causing the rest of  the  pattern  to
     fail. The classic example of where this gives problems is in
     trying to match comments in C programs. These appear between
     the  sequences /* and */ and within the sequence, individual
     * and / characters may appear. An attempt to  match  C  com-
     ments by applying the pattern

       //*.*/*/

     to the string

       /* first command */  not comment  /* second comment */

     fails, because it matches  the  entire  string  due  to  the
     greediness of the .*  item.

     However, if a quantifier is followed  by  a  question  mark,
     then it ceases to be greedy, and instead matches the minimum
     number of times possible, so the pattern

       //*.*?/*/

小结：
     ?与/U有类似功能，但同时出现彼此抵消

如下：
 $a = "asdf/*asdfaldsfasdf*/asfdasldf;kfldsj*/asfddsaf"; 
 $pattern = "*.*?/*///";
 //$pattern = "*.*/*///U";
 //$pattern = "*.*?/*///U";
 preg_match($pattern,$a,$match);
 print_r($match);
?>

 

2.Assertions
       /w+(?=;)

     matches a word followed by a semicolon, but does not include
     the semicolon in the match, and

       foo(?!bar)

     matches any occurrence of "foo"  that  is  not  followed  by
     "bar". Note that the apparently similar pattern

小结：
    (?!)只前向判断匹配，如bar(?!foo)，而(?!foo)bar没有意义
    (?