模式匹配-动态表达式的一个问题

QUOTE:

原帖由 HorseJia 于 2008-3-31 20:04 发表
示例一：
$_ = 'a2b';
/a(??{if($' eq '2b') {'2'}}/;

示例二：
$_ = 'a2b';
/a(??{if($' =~ m#2b#) {'2'}}/;

其中示例一，匹配成功；示例二，匹配不成功。
我想问的是，这意味着动态表达式的条件中 ...

CODE:

#!/usr/bin/perl
use re qw ( debug );
use strict;
use warnings;
$_ = "a2b";
#print "Matched at $& \n" if /a(?{ print "Goes here \n" if $' eq  "2b" ;})/;
print "Matched $& \n" if m@a(?(?{ $' =~ /2b/ })2)@;

QUOTE:

EXECUTING...

Guessing start of match, REx `a(?(?{ $' =~ /2b/ })2)' against `a2b'...
Found anchored substr `a' at offset 0...
Guessed: match at offset 0
Matching REx `a(?(?{ $' =~ /2b/ })2)' against `a2b'
  Setting an EVAL scope, savestack=12
   0 <> <a2b>             |  1:  EXACT <a>
   1 <a> <2b>             |  3:  LOGICAL[1]
   1 <a> <2b>             |  4:  EVAL
  re_eval 0x8080318
Guessing start of match, REx `2b' against `2b'...
Found anchored substr `2b' at offset 0...
Guessed: match at offset 0
Matching REx `2b' against `2b'
  Setting an EVAL scope, savestack=12
   0 <> <2b>              |  1:  EXACT <2b>
   2 <2b> <>              |  3:  END
Match successful!
-89031 <> <2b>              |15280:IFTHEN
-89031 <> <2b>              |15282:EXACT <2>
-89030 <> <b>              |15287:END
Match possible, but length=-89030 is smaller than requested=0, failing!
Match failed
Freeing REx: `"a(?(?{ $' =~ /2b/ })2)"'
Freeing REx: `"2b"'

QUOTE:

$_ = 'a2b';
/a(??{if($' eq '2b') {'2'}}/;