Question about character replacement with delayed environment variables - China DOS Union Forum

I recently wrote a script for character replacement with delayed environment variables, as follows:
@echo off
setlocal enabledelayedexpansion
set /p str=字符串：
set /p old=请输入old:
set /p new=请输入new：
set str=!str:!old!=!new!!
echo !str!
pause
But the result I get is oldnew, not the character replacement result I wanted. If I change the percent signs around old and new to exclamation marks, it works. Why? Wasn't it said that under delayed environment variables, variables have to be enclosed in exclamation marks? And the output result is also quite puzzling.

Floor1　lionlove	Posted 2010-08-15 11:18
新手上路　Posts 4　Credits 5
I recently wrote a script for character replacement with delayed environment variables, as follows: @echo off setlocal enabledelayedexpansion set /p str=字符串： set /p old=请输入old: set /p new=请输入new： set str=!str:!old!=!new!! echo !str! pause But the result I get is oldnew, not the character replacement result I wanted. If I change the percent signs around old and new to exclamation marks, it works. Why? Wasn't it said that under delayed environment variables, variables have to be enclosed in exclamation marks? And the output result is also quite puzzling.

Floor2　HAT	Posted 2010-08-15 13:11
版主　Posts 5,017　Credits 9,023
Variable nesting isn't used like this, refer to: http://bbs.bathome.net/thread-2899-1-1.html

Floor3　lionlove	Posted 2010-08-15 15:21
新手上路　Posts 4　Credits 5
After reading the reference materials, I understand a little. Could the moderator explain how variable nesting is used, and what needs attention when using variable nesting? Thanks.