Regular Expression is a set of expression which is used when we want to restrict the user to enter only particular values such as Phone number, Email, Address, pin code, etc.
Example: For an email Address text box I want to accept only email from the user like (name @ domain name) , and if the user enters the value other than the email format say only numbers or alphabets, then we want to display error message like "Please enter your email Address" , then in this situation we can use regular expression in order to avoid invalid data from the user.
Some of the most commonly used regular expressions are Phone no, Email id, Area name or Address,Pin code etc.
Collection of regular expressions which you can refer
here.
Thanks & I hope this helps you.