Description

The source code shows how to use Regular Expressions in C#. The code Functions written for Validation Alphabet, AlphaNumeric, Integer, Postive Integer, Floating point numbers. You just cut copy these functions and use in any program.

Explanation of Regular Expressions:

Regular expressions are used to search specified in the source string.

Examples:

Pattern#1
Regex objNotNaturalPattern=new Regex("[^0-9]");
Pattern#2 
Regex objNaturalPattern=new Regex("0*[1-9][0-9]*"); 

Pattern#1 will match for strings other than 0 to 9.^ symbol is used for Specifying not condition.[] brackets if we are to give range values such as 0 - 9 or a-z or A-Z 
eg. abc will return true

123 will return false.   

Pattern#2 will match for string which are Natural Numbers.Natural numbers Are numbers which are always greater than 0.The pattern 0* tells that a natural Number can be prefixed with any number of zero's or no zero's.the next [1-9] tells that it should contain atleast one number from 1 to 9 followed by any numbers of

0-9's 

Eg. 0007 returns true whereas 00 will return false. 

Basic things to be understood in RegEx: 

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

More RegEx patterns in Next Article. 

Source Code:

// 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