Expressions: Leading zeros and decimal places.

For some time now I have been cracking my head tryign to figure out if there is a way to develop an expression for Text Source that would allow the number (say, driven by a calculation, or a link to slider or another effect) to be presented with leading zeros.

Of course there is a possibility of including a large set of "if" statements, but I was hoping maybe someone knows an easier way.

Apart from leading zeros, how about displaying a number to a certain amount of decimal places. How about placing coma separator for thousands etc?

Say X=5738 Y=4

is there a simple way of making them display as:

5738 0004

or

5738.00 4.00

or

5,738 4

etc.