A validator is a computer program used to check the validity or syntactical correctness of a fragment of code or text.

IN ASP.NET there are six different types of Validators.

1.Required Field Validator.
2.Regular Expression Validator.
3.Compare Validator.
4.Range Validator.
5.CustomValidator.
6.Validation Summary

RequiredFieldValidator:-Ensure that the control has a value to assign in it or user does not skip an entry.

For Example:- 

 <asp:Textbox id="txtMobileNumber" runat="server"></asp:Textbox>
 <asp:RequiredFieldValidator id="RequiredFieldValidator1" runat="server"
 ControlToValidate="txtMobileNumber"
 ErrorMessage="Mobile Number is a required field."
 ForeColor="Red">
 </asp:RequiredFieldValidator>

Regular ExpressionValidator:-Ensure that the value of the control matches the expression of validator that is specified. This type of validation enables you to check for sequence of character, such as e-mail address, telephone number, postal code and so on.

For Example:- 

 <asp:TextBox ID="txtE-Mail" runat="server"></asp:TextBox>
 <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" 
 ControlToValidate="txtPassword"
 ErrorMessage="Please Enter a Valid E-Mail" 
 ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*"> // for internet E-Mail.
 </asp:RegularExpressionValidator>

CompareValidator:-Ensure that the value of one control should be equal to value of another control.which is commonly used in password,less than,greater than or equal to.

For Example:-

<asp:TextBox ID="txtPassword" runat="server"></asp:TextBox>
<asp:TextBox ID="txtConfirmPassword" runat="server"></asp:TextBox>
<asp:CompareValidator ID="CompareValidator1" runat="server" 
ControlToCompare="txtPassword" ControlToValidate="txtConfirmPassword" 
ErrorMessage="Password does not match">
</asp:CompareValidator>

RangeValidator:-Ensures that the value of the control is in specified lower and upper boundaries or specified range.  You can check ranges within pairs of numbers, alphabetic characters.

For Example:- 

<asp:TextBox ID="txtAmount" runat="server"></asp:TextBox>
<asp:RangeValidator ID="RangeValidator1" runat="server" 
ControlToValidate="txtPassword"
ErrorMessage="RangeValidator" MaximumValue="5000" 
MinimumValue="100">
</asp:RangeValidator>>

CustomValidator:-This control is used to perform user defined validations.

ValidationSummary:-This validator is used to displays a detailed summary on the validation errors that currently exist.

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