[ILUG] Shell substitute regexp

[Gavin McCullagh]

This is showing my ignorance, but very quickly..

if I have 10 files called 1.dat .. 10.dat

and I want to do 

	mv 1.dat 1
	...
	mv 10.dat 10

I know I can do the 

	for i in `ls` 
	do
	mv XXXXXXXXXXXXXXXXXXXXXXXXX
	done

but how do you construct the regexp to extract the first portion from the filename?  I could do it in perl, I just don't know what the syntax for using this is in the shell (specifically BASH).

thanks for your patience

Gavin