Regular expression In QTP :

How To Use : Regular expression to make our search globalize.

To execute the test in QTP every object is searched in the object repository (OR). In
real time applications some object's properties always keep changing so at the
time of execution script fails. To overcome it we use "regular expression " for
that particular object's property. For example:

1) Record a test where you are inserting one order and than faxing the same order.

2) The order number that is generated during recording may 21 and Fax Order window title is generated and saved into OR as Fax Order No. 21

3) Now execute the same test the script will fail because this time order no is modified as 22 and accordingly Fax Order window title is modified as Fax Order No. 22 .

4) This time the object Fax Order No. 22 is not identified or you can understand object is not matched (find) into OR.

5) To make this search better we modify the text property of object into OR as Fax Order No.* this will search all the
occurrences of window that has text as Fax Order No

6) We can also write it as Fax Order No. [0-9] because fax order no is a numerical value and this will search only numbers after Fax Order No.

So this is the purpose of regular expression in QTP.

To handle the dynamics objects either window/web based application. To handle the objects those name having string along with number the number may change in every execution or every particular time. To handle these objects we can use regular expression.
It have 5 types
1. Dot - .
2. regular expression
3. Star - *
4. Carat - ^
5. Range - [ ]

Also Regular Exp is used for pattern matching and replacing of a string.

* : Matches the preceding character zero or more times.
. : Matches any single character except a newline character.

For gaining more insights in the automation using QTP log on to below url :

Automation Testing Using QTP