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Modern C# and .NET Tutorial: Cross-Platform Basics

Pattern Matching with Regular Expressions

Learn about regular expressions to validate user input, understand their syntax, and efficiently use them for tasks like splitting complex comma-separated strings.

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Regular expressions are useful for validating input from the user. They are very powerful and can get very complicated. Almost all programming languages support regular expressions and use a common set of special characters to define them. Let’s try out some example regular expressions:

Step 1: Use your preferred code editor to add a new Console App or console project named WorkingWithRegularExpressions to the Chapter08 solution or workspace:

  • In Visual Studio Code, select WorkingWithRegularExpressions as the active OmniSharp project.

Step 2: In Program.cs, delete the existing statements, and then import the following namespace:

C#
using System.Text.RegularExpressions; // Regex

Checking for digits entered as text

We will start by implementing the common example of validating number input:

Step 1: In Program.cs, add statements to prompt the user to enter their age and then check that it is valid using a regular expression that looks for a digit character, as shown in the following code:

C#
Write("Enter your age: ");
string input = ReadLine()!; // null-forgiving
Regex ageChecker = new(@"\d");
if (ageChecker.IsMatch(input))
{
  WriteLine("Thank you!");
}
else
{
  WriteLine($"This is not a valid age: {input}");
}

Note the following about the code:

  • The @ character switches off the ability to use ...