#! /usr/bin/perl #
 $a="this is a match THIS";
 $a=~/(TH)/;
 print " first $1\n";
 $a=~/(THIS)/;
print "second $1\n";
 $a=~/(THAT)/;
print "third $1\n";

程序运行结果：
first TH
second THIS
thrid THIS

为什么最后一次匹配不到THAT，$1返回的不是空而是上次匹配成功的？