My system: Windows Xp SP3(32bit), ntEmacs 23.0.0.1 (i386-mingw-nt5.1.2600)
 
My file's content is like below. 
Notice that the first character of the first line is a whitespace, 
and the first character of the second line is not a whitespace.
 
 This is my text file.
This is my text file.
 
 
Aaginst this text, I gave a command.
 
M-x align-regexp RET
 
When minibuffer shows "Align regexp: ", I just struck Retern key without giving any regular expression.
And then Emacs doesn't response any more. Is this a bug?
 
Reproduction scenario
1. The first character of your file's first line begins a whitespace.
2. The first character of your file's second line begins a non-whitespace character.
3. After the M-x align-regexp command, just strike Return key without giving any regular expression.
 
Result: Emacs doesn't response any more. You should kill the process of Emacs.
 
Good luck to you all. Long live GNU, Linux, Emacs!! :-)