regex question

[Roy Smith]

This isn't really a python question per-se, but it came up while using 
the re module, so there is a connection :-)

Let's say I've got a pattern "[0-9]", i.e. a 1-digit number.  I want to 
search a string for that pattern, and only return a match if it's found 
preceeded by either a non-digit or the beginning of the string, and 
likewise followed by either a non-digit or the end of the string.  Is 
there a single compact regex I can write to cover that?

For example:

pat = re.compile (pattern)
x = pat.search ("1")
y = pat.search ("12")
z = pat.search ("the number 1, don't you know?")

x and z should be MatchObjects, but y should be None.

The best I can come up with would be something like:

(^[0-9][^0-9])|([^0-9][0-9][^0-9])|([^0-9][0-9]$)|(^[0-9]$)

which is fairly ugly.  This uglyness is exaccerbated by the fact that 
I'm not really looking for 1-digit strings, I'm looking for 12-digit hex 
numbers (i.e. ethernet MAC addresses), where the basic pattern is:

[0-9a-fA-f][0-9a-fA-f][0-9a-fA-f][0-9a-fA-f][0-9a-fA-f][0-9a-fA-f][0-9a-f
A-f][0-9a-fA-f][0-9a-fA-f][0-9a-fA-f][0-9a-fA-f][0-9a-fA-f].