the following should -not- match:

$ python
Python 2.1b1 (#1, Mar 12 2001, 18:20:53) 
[GCC 2.95.2 20000220 (Debian GNU/Linux)] on linux2
Type "copyright", "credits" or "license" for more
information.
>>> reg = r"(?im)<dtml-var\s+([a-z_0-9]+?)\s*>"
>>> str = '<dtml-var
expr="Presentation.show(\'start\')">'
>>> import re                                
>>> re.match(reg, str)                       
<SRE_Match object at 0x810d9d0>


In python 1.5.2 and 2.0 this works fine.