How to validate empty text box field using regural expression?

You need to use DOM to get the contents.
You need to use php or javaScript to analyze the contents by whatever rules you use to define 'empty'
You need to then handle the result (true/false, for example).

Beginner?
If you are in school, you should seize the struggle so as to learn. Create tests. See what they do. Develop methods that you can (personally) use to solve problems.

For an empty field you do NOT need regular expression, simply write (field.value == null)

The expression above, if I remember my rusty regex, takes 1 to 20 letters, no spaces. Google it, its not hard.