Regular expressions, commonly referred to as Regex, are a powerful tool for text parsing, searching, and manipulation. In C#, the System.Text.RegularExpressions namespace provides the Regex class, which allows you to work efficiently with regular expressions. This tutorial will walk you through the basics of regex in C#, the various methods and properties available, with examples and practical use cases to help you master using regular expressions.
Regular expressions are patterns used to match character combinations in strings. They can be used for a variety of text-processing tasks, such as validation, searching, and replacing text.
C#'s Regex class in the System.Text.RegularExpressions namespace provides robust capabilities to implement regex-based operations seamlessly. By learning how to leverage the Regex class, you can efficiently perform string manipulations that might otherwise require cumbersome looping and conditional logic.
The syntax of regex involves a combination of characters that form a search pattern. Here are some common regex patterns:
.: Matches any character.*: Matches zero or more occurrences of the preceding character.+: Matches one or more occurrences of the preceding character.[a-z]: Matches any lowercase letter.\d: Matches any digit (0-9).^: Matches the start of a string.$: Matches the end of a string.|: Logical OR.^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$^\d{3}-\d{3}-\d{4}$ (Matches a phone number in 123-456-7890 format)The Regex class in C# provides various methods to perform operations on text. Let’s explore these methods and properties.
Options: Gets the options that were passed to the Regex object.
Regex regex = new Regex(@"\d+", RegexOptions.IgnoreCase);
Console.WriteLine(regex.Options); // Output: IgnoreCase
MatchTimeout: Gets the maximum time interval to execute a match.
Regex regex = new Regex(@"\d+", RegexOptions.None, TimeSpan.FromSeconds(1));
Console.WriteLine(regex.MatchTimeout); // Output: 00:00:01
IsMatch(string): Checks if the regex matches a part of the given input string.
Regex regex = new Regex(@"\d{3}-\d{3}-\d{4}");
bool isMatch = regex.IsMatch("123-456-7890");
Console.WriteLine(isMatch); // Output: True
Match(string): Searches an input string for a match.
Regex regex = new Regex(@"\d+");
Match match = regex.Match("My number is 42");
Console.WriteLine(match.Value); // Output: 42
Matches(string): Returns all matches within an input string.
Regex regex = new Regex(@"\d+");
MatchCollection matches = regex.Matches("Numbers: 42, 56, 78");
foreach (Match match in matches)
{
Console.WriteLine(match.Value); // Output: 42, 56, 78
}
Replace(string, string): Replaces all matches within an input string with a specified replacement.
Regex regex = new Regex(@"\d+");
string result = regex.Replace("I have 2 apples and 3 oranges", "many");
Console.WriteLine(result); // Output: I have many apples and many oranges
Split(string): Splits an input string based on the regex pattern.
Regex regex = new Regex(@"\s+");
string[] words = regex.Split("This is a test");
foreach (string word in words)
{
Console.WriteLine(word); // Output: This, is, a, test
}
To validate a phone number in the format 123-456-7890, you can use the following code:
using System.Text.RegularExpressions;
class Program
{
static void Main()
{
Regex phoneRegex = new Regex(@"^\d{3}-\d{3}-\d{4}$");
string phoneNumber = "123-456-7890";
if (phoneRegex.IsMatch(phoneNumber))
{
Console.WriteLine("Valid phone number");
}
else
{
Console.WriteLine("Invalid phone number");
}
}
}
^\d{3}-\d{3}-\d{4}$ checks for three sets of digits separated by hyphens.IsMatch checks if the phone number matches this pattern.using System.Text.RegularExpressions;
class Program
{
static void Main()
{
Regex regex = new Regex(@"\d+");
string input = "The order numbers are 123, 456, and 789.";
MatchCollection matches = regex.Matches(input);
foreach (Match match in matches)
{
Console.WriteLine($"Found number: {match.Value}");
}
}
}
\d+ matches one or more digits.Matches returns all the matches, which are then printed.using System.Text.RegularExpressions;
class Program
{
static void Main()
{
Regex regex = new Regex(@"\d");
string input = "Password123!";
string replaced = regex.Replace(input, "*");
Console.WriteLine(replaced); // Output: Password***
}
}
\d matches any digit.Replace replaces all digits with *.using System.Text.RegularExpressions;
class Program
{
static void Main()
{
Regex regex = new Regex(@"\W+");
string input = "Hello, world! Welcome to C#.";
string[] words = regex.Split(input);
foreach (string word in words)
{
Console.WriteLine(word);
}
}
}
Explanation:
A common use of regex is to validate email addresses. Let’s look at an example that demonstrates email validation.
The pattern for validating emails is ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$.
^[a-zA-Z0-9._%+-]+: Matches the local part of the email.@: Matches the @ symbol.[a-zA-Z0-9.-]+: Matches the domain name.\.[a-zA-Z]{2,}$: Matches the top-level domain (e.g., .com, .org).using System;
using System.Text.RegularExpressions;
class Program
{
static void Main()
{
Regex emailRegex = new Regex(@"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$");
string email = "test@example.com";
if (emailRegex.IsMatch(email))
{
Console.WriteLine("Valid email address");
}
else
{
Console.WriteLine("Invalid email address");
}
}
}
IsMatch method checks if the email matches the specified pattern.Use Case: This type of validation is widely used in web forms, sign-up pages, and any system requiring user input verification to ensure data consistency.
IsMatch: Check if a pattern matches a string.Match and Matches: Extract specific information.Replace: Replace matched text with a specified string.Split: Split strings based on a pattern.The Regex class in C# is a robust tool for text processing tasks such as searching, extracting, validating, and replacing text. By mastering the methods and properties provided by the Regex class, you can handle complex string operations easily, enhancing both the readability and maintainability of your code.
We covered the most commonly used methods (IsMatch, Match, Replace, Split) and properties of the Regex class, along with several practical examples to demonstrate their use. Whether you're validating user input like email addresses, extracting numbers, or splitting sentences into words, regex provides a versatile solution for a wide range of programming challenges. With these capabilities, you can write code that efficiently manages text-related tasks in C#.