Regular Expressions Basics

Introduction

A regular expression (or the acronym "regex") is a pattern that allows for complex text searches.

Imagine you need to search for e-mail addresses in a given text. An e-mail address has a complex structure: it has an account ID part, the @ sign, the domain name part. On top of that, each part has its own restrictions.

This is a hard task to do if you try to program it as usual (with loops, simple text searches and conditionals).

Instead of that, let's try to search for e-mail addresses with a regular expression.

Here is an example Regex that matches e-mail addresses:

Everything that is before the @ is the E-mail ID, and everything after the @ is the E-mail Domain Name.

The example below provides some code with an e-mail regex pattern. To test it, press RUN MY CODE

Example 1 - Searching E-mail addresses

If you click on the View Validators button, you'll see what has been matched in each validator (shown by a colored highlight). Use the Hints button if you ever get stuck on an Exercise. You can also use the Show me the Solution! to get a working solution for the problem. Read the comments carefully on each exercise, as it gives instructions to solve it.

As you can see, a regex pattern is a very powerful tool. It's used in a lot of different fields and programs.

In this course you'll learn the basics of regex: enough to understand a regex pattern and to create your own patterns.

On C# literal strings

I will use C# as the reference language for the course. Please note that string definition varies in other languages. It's important that you take into account how your language treats some characters inside strings. For example, C# doesn't take the \ character as literal in normal strings. This is problematic for regex, because then you'll need to escape \.

For this reason, I use string literals in C# (see https://msdn.microsoft.com/en-us/library/aa691090(v=vs.71).aspx). C# string literals are defined with a @ before the quote:

 string literal = @"This is a verbatim string literal, \ doesn't need to be escaped";
 string  normal =  "This is a normal string, \\ needs to be escaped";

If you use another language, you must ensure that your regex pattern is literal. Otherwise you'll get unexpected results, or you'll need to escape non-literal characters.

In the next lesson, you'll learn about reserved characters in Regular Expressions.