What are regular expressions? These are patterns that help us work with text. Regular expressions (regex, regexp) used to match sequences of characters in strings: search, edit, delete any area in large texts, data generation or validation, and so on. Regexes can be used in programming languages, command line interfaces, etc...

For example I want validate email addresses from my domain. The expression will look like this:

^.*\@example\.com$

^ symbol means the beginning of an expression.
Next symbols .* need for matching any address in @example.com domain.
Backslash before "at" symbol need for validate @ as is. The same situation with \. symbols.
$ - end of expression.

Result:

Next case need for validation phone numbers. I need accept numbers only from my region. In my case numbers started from +77. I should use this expression:

^\+77\d{1,9}$

^ symbol means the beginning of an expression.
\+ - using "plus" symbol as "plus" symbol.
\d - presents d as digits.
In curly brackets {1,9} digits length.
$ - end of expression.

Result:

Additionally maybe validation when phone number starts from "+77" or "87":

^(\+77|87).?\d{1,9}$

(\+77|87).? - number starts from +77 or 87.

Result:

Now let's look at IP address validation. For verification IP addresses you may use this expression:

((25[0-5]|(2[0-4]|1\d|[1-9]|)\d)\.?\b){4}

Result:

Thanks for regex101.com site, which allowed test my regular expressions.