In QTP, delaing of Regualar Expressions and varing object 
descriptions can be handled by using Description 
Programming.
Ex. of using Regular expression is:
u just record yahoo login operaion and click on Inbox link. 
Assume u got 10 new mails, then it shows like Inbox(10). 
Read one mail, now it shows like Inbox(9). Now try to play 
the recorded script. It wont work. Now use Regular 
expression in Inbox link like:
"Inbox.*". At that time it wil work.

Thax uday to give valuable information about regular 
expression.

Regular expressions enable QuickTest to identify objects 
and text strings with varying values.You can use regular 
expressions only for values of type string.

You can use regular expressions when: 

1) Defining the property values of an object in dialog 
boxes or in programmatic descriptions.

2) Parameterizing a step.

3) Creating checkpoints with varying values.

Example:

your site may include a form in which the user inputs data 
and clicks the Send button to submit the form. When a 
required field is not completed, the form is displayed 
again for the user to complete. When resubmitting the form, 
the user clicks the Resend button. You can define the value 
of the button's name property as a regular expression, so 
that QuickTest ignores variations in the button name when 
clicking the button.

ex
if we take flight reservation table (coz its easy to
understand the concept)
just record the script like this 
go to file open order give no 5
and go to fax order type any fax no click send
and stop recording
then go to qtp script just change the script (the order no
to 6 )
Window("Flight Reservation").Dialog("Open
Order").WinEdit("Edit").Set "6"
and run the script we get the error as qtp could not
identify the fax order no 5
then go to repository select fax order no change the value
to  "fax order no. *"
run it u get  it

Thanks Mr.Uday u explained beautyfully with an example. Can 
u give another two more real time secnarios where we use RE.