关于perl正则表达式全局匹配的一个奇怪问题

gaojianchun1984

代码实例如下，当使用全局匹配/g选项时，发现第二个if匹配块不起作用，取消/g选项后恢复正常，请高手帮忙讲解一下原因，多谢！我用的是perl 5.22.1版本。
带/g：

1. use strict;
   
2. use warnings;
   
3. 
   
4. my $str = "ab";
   
5. 
   
6. if($str =~ m/b/g)
   
7. {
   
8.     print("find b\n");
   
9. }
   
10. 
    
11. if($str =~ m/a/g)
    
12. {
    
13.     print("find a\n");
    
14. }

复制代码

程序输出："find b"

取消/g：

1. use strict;
   
2. use warnings;
   
3. 
   
4. my $str = "ab";
   
5. 
   
6. if($str =~ m/b/)
   
7. {
   
8.     print("find b\n");
   
9. }
   
10. 
    
11. if($str =~ m/a/)
    
12. {
    
13.     print("find a\n");
    
14. }

复制代码

程序输出："find b  find a"

jason680

回复 1# gaojianchun1984


if($str =~ m/b/g){ ... }    # note: ab
  found the "b" and keep the position 2 (after b)

if($str =~ m/a/g){ ... }    # note: ab
  cannot find the "a" after "b" (postion 2)



$ perldoc perlretut
NAME
    perlretut - Perl regular expressions tutorial

   ...

   Global matching
    The final two modifiers we will discuss here, "//g" and "//c", concern
    multiple matches. The modifier "//g" stands for global matching and allows
    the matching operator to match within a string as many times as possible.
    In scalar context, successive invocations against a string will have "//g"
    jump from match to match, keeping track of position in the string as it
    goes along. You can get or set the position with the "pos()" function.


   

gaojianchun1984

多谢大神指点！

sunzhiguolu

这个问题我也是稀里糊涂的, 今天总是明白了. 谢谢大神指点.