Regex in C# defines a regular expression in C#. The Regex class offers methods and properties to parse a large text to find patterns of characters. In this article, you’ll learn how to use a Regex class in C#.
Regular Expressions in C#
A regular expression is used to check whether a string matches a pattern. C# regex, also known as C# regular expression or C# regexp, is a sequence of characters that defines a pattern. A pattern may consist of literals, numbers, characters, operators, or constructs. The pattern searches strings or files to see if matches are found. Regular expressions are often used in input validations, parsing, and finding strings. For example, checking a valid date of birth, social security number, and full name where the first and the last names are separated by a comma, finding a number of occurrences of a substring, replacing substrings, date formats, valid email formats, a currency format, and so on.
1. C# Regex Class
C# Regex class represents the regular expression engine. It can quickly parse large amounts of text to find specific character patterns; extract, edit, replace, or delete text substrings; and add the extracted strings to a collection to generate a report.
In .NET, the Regex class is defined in the System.Text.RegularExpressions namespace. The Regex class constructor takes a pattern string as a parameter with other optional parameters.
The following code snippet creates a Regex from a pattern. Here the pattern is to match a word starting with char ‘M’.
// Create a pattern for a word that starts with the letter "M"
string pattern = @"\b[M]\w+";
// Create a Regex
Regex rg = new Regex(pattern);
The following code snippet has a long text with author names that must be parsed.
// Long string
string authors = "Mahesh Chand, Raj Kumar, Mike Gold, Allen O'Neill, Marshal Troll";
The Matches method finds all the matches in a Regex and returns a MatchCollection.
// Get all matches
MatchCollection matchedAuthors = rg.Matches(authors);
The following code snippet loops through the matches collection.
// Print all matched authors
for (int count = 0; count < matchedAuthors.Count; count++)
Console.WriteLine(matchedAuthors[count].Value);
Here is the complete code:
// Create a pattern for a word that starts with the letter "M"
string pattern = @"\b[M]\w+";
// Create a Regex
Regex rg = new Regex(pattern);
// Long string
string authors = "Mahesh Chand, Raj Kumar, Mike Gold, Allen O'Neill, Marshal Troll";
// Get all matches
MatchCollection matchedAuthors = rg.Matches(authors);
// Print all matched authors
for (int count = 0; count < matchedAuthors.Count; count++)
Console.WriteLine(matchedAuthors[count].Value);
In the above example, the code looks for char ‘M’. But what if the word starts with ‘m’? The following code snippet uses RegexOptions.IgnoreCase parameter to ensure that Regex does not look for uppercase or lowercase.
// Create a pattern for a word that starts with letter "M"
string pattern = @"\b[m]\w+";
// Create a Regex
Regex rg = new Regex(pattern, RegexOptions.IgnoreCase);
2. Replacing multiple white spaces using Regex
The Regex.Replace() method replaces a matched string with a new one. The following example finds multiple whitespaces in a string and replaces them with a single whitespace.
// A long string with a ton of white spaces
string badString = "Here is a strig with ton of white space." ;
string CleanedString = Regex.Replace(badString, "\\s+", " ");
Console.WriteLine($"Cleaned String: {CleanedString}");
The following code snippet replaces whitespaces with a ‘-‘.
string CleanedString = Regex.Replace(badString, "\\s+", "-");
3. Replacing multiple white spaces using Regex in C#
The following example uses the regular expression pattern [a-z]+ and the Regex.Split() method to split a string on any uppercase or lowercase alphabetic character.
// Spilt a string on alphabetic character
string azpattern = "[a-z]+";
string str = "Asd2323b0900c1234Def5678Ghi9012Jklm";
string[] result = Regex.Split(str, azpattern,
RegexOptions.IgnoreCase, TimeSpan.FromMilliseconds(500));
for (int i = 0; i < result.Length; i++)
{
Console.Write("'{0}'", result[i]);
if (i < result.Length - 1)
Console.Write(", ");
}
Regular Expressions in C#
Regular expressions are a pattern matching standard for string parsing and replacement. They are a way for a computer user to express how a program should look for a specified pattern in text and what the program should do when each pattern match is found. Sometimes it is abbreviated "regex". They are a powerful way to find and replace strings in a defined format.
Here is a simple code example in C# that shows how to use regular expressions.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
namespace RegularExpression1
{
class Program
{
static void Main(string[] args)
{
Regex r = new Regex(@"^\+?\d{0,2}\-?\d{4,5}\-?\d{5,6}");
//class Regex represents an immutable regular expression.
string[] str = { "+91-9678967101", "9678967101", "+91-9678-967101", "+91-96789-67101", "+919678967101" };
//Input strings for Match valid mobile number.
foreach (string s in str)
{
Console.WriteLine("{0} {1} a valid mobile number.", s,
r.IsMatch(s) ? "is" : "is not");
//The IsMatch method is used to validate a string or
//to ensure that a string conforms to a particular pattern.
}
}
}
}
Here is a detailed explanation of Regular Expressions and how to use them in C# and .NET:
Regular Expressions in C#
4. Regex for Email Validation in C#
For validating multiple emails, we can use the following regular expressions. We are separating emails by using the delimiter ';'
^((\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*)\s*[;]{0,1}\s*)+$
If you want to use delimiter ',' then use this.
^((\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*)\s*[,]{0,1}\s*)+$
And if you want to use both delimiters ',' and ';,' then use this.
^((\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*)\s*[;,.]{0,1}\s*)+$
So by using the above regular expression, you can validate a single email as well as multiple emails.
Learn more here: Regex for Multiple Email Validation.
5. Validating User Input With Regular Expressions in C#
This article explains how to use Regular expressions (the Regx class of the System.Text.RegularExpressions namespace) in C# and .NET.
We can use Regex.Match method that takes an input and regex and returns success if
if (!Regex.Match(firstNameTextBox.Text, "^[A-Z][a-zA-Z]*$").Success) {}
if (!Regex.Match(addressTextBox.Text, @"^[0-9]+\s+([a-zA-Z]+|[a-zA-Z]+\s[a-zA-Z]+)$").Success)
if (!Regex.Match(cityTextBox.Text, @"^([a-zA-Z]+|[a-zA-Z]+\s[a-zA-Z]+)$").Success)
if (!Regex.Match(stateTextBox.Text, @"^([a-zA-Z]+|[a-zA-Z]+\s[a-zA-Z]+)$").Success)
if (!Regex.Match(zipCodeTextBox.Text, @"^\d{5}$").Success)
{
if (!Regex.Match(phoneTextBox.Text, @"^[1-9]\d{2}-[1-9]\d{2}-\d{4}$").Success)
Learn more here:
Validating User Input with Regular Expressions
6. Split String using Regex.split (Regular expression) in C#
In this post, we will learn how to split the string using Regex in c#.
Here we will learn how to split the string using RegEx in C#. Regex splits the string based on a pattern. It handles a delimiter specified as a pattern. This is why Regex is better than string.Split. Here are some examples of how to split a string using Regex in C#. Let's start coding.
For use, Regex adds the below namespaces for splitting a string.
using System;
using System.Text.RegularExpressions;
using System.Collections.Generic;
Example 1
Split digits from a string using Regex.
string Text = "1 One, 2 Two, 3 Three is good.";
string[] digits = Regex.Split(Text, @"\D+");
foreach (string value in digits)
{
int number;
if (int.TryParse(value, out number))
{
Console.WriteLine(value);
}
}
The above code splits the string using \D+ and loops through check number and print.
Learn more here:
Split String using Regex in C#
7. Replace Special Characters from string using Regex in C#
Learn how to replace Special Characters Using Regex in C#.
You can use regex if you have a string with special characters and want to remove/replace them.
Use this code:
Regex.Replace(your String, @"[^0-9a-zA-Z]+", "")
This code will remove all of the special characters, but if you don't want to remove some of the special characters for, e.g., comma "," and colon ":" then make changes like this:
Regex.Replace(Your String, @"[^0-9a-zA-Z:,]+", "");
Similarly, you can make changes according to your requirement.
Note:
Please note that regular expressions do not solve every tiny string parsing. If you need simple parsing provided by the String class or other classes, try to use those.