关于 =~ s/../../g 转换的问题

Perl_Er

$cc = "$3$2$1"; 这个会取$3,$2,$1的值,因为他们都是undef,$cc为空，所以你替换后是空的
试下下面的代码，打印一下$1$2$3.

1. $aa = "<a>asfsafasdfsa<c>";
   
2. $ab = "(<a>)(.*)(<c>)";
   
3. $cc = "$3$2$1";
   
4. [color=Red]print $1;
   
5. print $2;
   
6. print $3;[/color]
   
7. $aa =~ s/$ab/$cc/g;
   
8. print "$aa\n";

复制代码

[ 本帖最后由 Perl_Er 于 2009-7-28 09:02 编辑 ]

Perl_Er

加这些是为了打印的值,debug.

Perl_Er

/e是把//内的当作一段代码执行，第一个里面没有操作符，想一下假设$a$b$c放在一起执行会不会有问题，中间没有操作符连接，
第二个就不用说了，用.操作符连接了。第三个的话吧$1$2$3一起当作了一个字符串。

Perl_Er

就像 $c='$1', $c的值不就是 一个 $ 和一个 1组成的字符串吗？

这是第一次，还有第二次了。
先休息一下吧。晕了想不清楚的

Perl_Er

With an eval, you should be especially careful to remember what's being looked at when:

   1. eval $x; # CASE 1
   2. eval "$x"; # CASE 2
   3.
   4. eval '$x'; # CASE 3
   5. eval { $x }; # CASE 4
   6.
   7. eval "\$$x++"; # CASE 5
   8. $$x++; # CASE 6

Cases 1 and 2 above behave identically: they run the code contained in the variable $x. (Although case 2 has misleading double quotes making the reader wonder what else might be happening (nothing is).) Cases 3 and 4 likewise behave in the same way: they run the code '$x' , which does nothing but return the value of $x. (Case 4 is preferred for purely visual reasons, but it also has the advantage of compiling at compile-time instead of at run-time.) Case 5 is a place where normally you would like to use double quotes, except that in this particular situation, you can just use symbolic references instead, as in case 6.