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