in ASP.net to validate user checking, it's more convenient way then just
doint it with list of if's
in other cases to find smth, to replace test by template (for exaple all
numbers or smth) ant etc

Regular Expressions are used for finding character patterns in text. this is
entirely different from finding individual characters or substrings in text.
For example, you might want to find all <img> tags in an HTML document. Now,
each of these tags has some things in common. For example, they all begin
with "<img". They all end with ">". But what's in-between the beginning and
the end of the tag can be entirely different. Regular Expressions can match
all substrings in a string that match a pattern, or a set of rules about the
text in the substring.

There is a whole language of symbols for looking for different types of
patterns in text. The Regular Expressions classes are classes for working
with the Regular Expressions language, and performing different types of
operations with it.

This is a notation that allows to describe a "pattern" (such as "3 digits
followed by two optional digits then the "-" symbol and either the A or B
letter"). You can then use this expression to see if a string match this
pattern (you can also search and even search/replace those patterns).