hwen ever change in regular in test objects that tine use 
resgular expression

Regular exprresion is used where varying window lables are 
there at run time 

for ex. while recording script on MS document , it was doc-2

but when u run the script the window lable changes to doc-3
 then QTp doesnt identify this ...
for this REgular expression is needed

In flight application, fax order window number changes 
according to the Order number created.....
in this case u can use regular expression for that number.. 
so that the test will not fail and it continues whatever be 
the number in that window..

Regular expression are used for the dynamic values. Dynamic 
values change there value during the run time.
for ex. date, When we record the application it take the 
default date and when we run the test on different day it 
will select that day date. For this conditions it's better 
to use the regulat expressions.

Regular Expressions can be used for Dynamically chanaging 
objects in the run time. 

In QTP we use regular expressions in

1. Physical description 
2. Check points
3. Recovery Scenarios
4. In Scripting