Regular Expressions in C#

Remember to escape special characters in the pattern string using a backslash (\) if you want to match those characters literally.

Regular expressions in C# provide a powerful and flexible way to work with text data, allowing you to search, validate, and transform strings efficiently.

Detailed Regex Example

Let's create a complete application with a few patterns.

Pattern #1:

Regex objNotNaturalPattern=new Regex("[^0-9]");

Pattern #2:

Regex objNaturalPattern=new Regex("0*[1-9][0-9]*");

Pattern #1 will match strings other than 0 to 9.

• The ^ symbol is used to specify, not condition.
• the [] brackets if we are to give range values such as 0 - 9 or a-z or A-Z

In the above example, input 'abc' will return true, and '123' will return false.

Pattern #2 will match strings that are natural numbers. Natural numbers are numbers that are always greater than 0. Pattern 0* says a natural number can be prefixed with zeros or non-zero. The next [1-9] says it should contain at least one number from 1 to 9 followed by any numbers of 0-9's.

In this case, input '0007' will return true, and '00' will return false.

Basic operators to understand in Regex:

• "*" matches 0 or more patterns
• "?" matches a single character
• "^" for ignoring matches.
• "[]" for searching range patterns.

The complete source code example provides functions to check IsNaturalNumber, IsWholeNumber, IsPositiveNumber, IsInteger, IsNumber, IsAlpha, and IsAlphaNumeric.

// Source Code starts  
using System.Text.RegularExpressions;  
using System;  
/* 
<HowToCompile> 
csc /r:System.Text.RegularExpressions.dll,System.dll Validation.cs  
</HowToComplie> 
*/  
class Validation  
{  
    public static void Main()  
    {  
        String strToTest;  
        Validation objValidate = new Validation();  
        Console.Write("Enter a String to Test for Alphabets:");  
        strToTest = Console.ReadLine();  
        if (objValidate.IsAlpha(strToTest))  
        {  
            Console.WriteLine("{0} is Valid Alpha String", strToTest);  
        }  
        else  
        {  
            Console.WriteLine("{0} is not a Valid Alpha String", strToTest);  
        }  
    }  
    // Function to test for Positive Integers.   
    public bool IsNaturalNumber(String strNumber)  
    {  
        Regex objNotNaturalPattern = new Regex("[^0-9]");  
        Regex objNaturalPattern = new Regex("0*[1-9][0-9]*");  
        return !objNotNaturalPattern.IsMatch(strNumber) &&  
        objNaturalPattern.IsMatch(strNumber);  
    }  
    // Function to test for Positive Integers with zero inclusive   
    public bool IsWholeNumber(String strNumber)  
    {  
        Regex objNotWholePattern = new Regex("[^0-9]");  
        return !objNotWholePattern.IsMatch(strNumber);  
    }  
    // Function to Test for Integers both Positive & Negative   
    public bool IsInteger(String strNumber)  
    {  
        Regex objNotIntPattern = new Regex("[^0-9-]");  
        Regex objIntPattern = new Regex("^-[0-9]+$|^[0-9]+$");  
        return !objNotIntPattern.IsMatch(strNumber) && objIntPattern.IsMatch(strNumber);  
    }  
    // Function to Test for Positive Number both Integer & Real   
    public bool IsPositiveNumber(String strNumber)  
    {  
        Regex objNotPositivePattern = new Regex("[^0-9.]");  
        Regex objPositivePattern = new Regex("^[.][0-9]+$|[0-9]*[.]*[0-9]+$");  
        Regex objTwoDotPattern = new Regex("[0-9]*[.][0-9]*[.][0-9]*");  
        return !objNotPositivePattern.IsMatch(strNumber) &&  
        objPositivePattern.IsMatch(strNumber) &&  
        !objTwoDotPattern.IsMatch(strNumber);  
    }  
    // Function to test whether the string is valid number or not  
    public bool IsNumber(String strNumber)  
    {  
        Regex objNotNumberPattern = new Regex("[^0-9.-]");  
        Regex objTwoDotPattern = new Regex("[0-9]*[.][0-9]*[.][0-9]*");  
        Regex objTwoMinusPattern = new Regex("[0-9]*[-][0-9]*[-][0-9]*");  
        String strValidRealPattern = "^([-]|[.]|[-.]|[0-9])[0-9]*[.]*[0-9]+$";  
        String strValidIntegerPattern = "^([-]|[0-9])[0-9]*$";  
        Regex objNumberPattern = new Regex("(" + strValidRealPattern + ")|(" + strValidIntegerPattern + ")");  
        return !objNotNumberPattern.IsMatch(strNumber) &&  
        !objTwoDotPattern.IsMatch(strNumber) &&  
        !objTwoMinusPattern.IsMatch(strNumber) &&  
        objNumberPattern.IsMatch(strNumber);  
    }  
    // Function To test for Alphabets.   
    public bool IsAlpha(String strToCheck)  
    {  
        Regex objAlphaPattern = new Regex("[^a-zA-Z]");  
        return !objAlphaPattern.IsMatch(strToCheck);  
    }  
    // Function to Check for AlphaNumeric.  
    public bool IsAlphaNumeric(String strToCheck)  
    {  
        Regex objAlphaNumericPattern = new Regex("[^a-zA-Z0-9]");  
        return !objAlphaNumericPattern.IsMatch(strToCheck);  
    }  
}  
// Source Code End 

Here are some of the most common Regex code examples in C#:

C# Regex Examples (2023)