how to validate textbox in asp.net

You can done this in following ways:
1.AjaxFilteredTextBoxExtender
 
here you first add reference of ajax toolkit and this extender.
you can set ValidChars to "." for decimal which allo user to enter numeric value with decimal point.

<ajaxToolkit:FilteredTextBoxExtender ID="ftbe" runat="server"
    TargetControlID="TextBox3"
    FilterType="Custom, Numbers"
    ValidChars="." />

 
2.using javscript
 
<script type="text/javascript">
// price text-box allow numeric and allow 2 decimal points only
function extractNumber(obj, decimalPlaces, allowNegative)
{
var temp = obj.value;

// avoid changing things if already formatted correctly
var reg0Str = '[0-9]*';
if (decimalPlaces > 0) {
reg0Str += '\\.?[0-9]{0,' + decimalPlaces + '}';
} else if (decimalPlaces < 0) {
reg0Str += '\\.?[0-9]*';
}
reg0Str = allowNegative ? '^-?' + reg0Str : '^' + reg0Str;
reg0Str = reg0Str + '$';
var reg0 = new RegExp(reg0Str);
if (reg0.test(temp)) return true;
 
// first replace all non numbers
var reg1Str = '[^0-9' + (decimalPlaces != 0 ? '.' : '') + (allowNegative ? '-' : '') + ']';
var reg1 = new RegExp(reg1Str, 'g');
temp = temp.replace(reg1, '');
 
if (allowNegative) {
// replace extra negative
var hasNegative = temp.length > 0 && temp.charAt(0) == '-';
var reg2 = /-/g;
temp = temp.replace(reg2, '');
if (hasNegative) temp = '-' + temp;
}

if (decimalPlaces != 0) {
var reg3 = /\./g;
var reg3Array = reg3.exec(temp);
if (reg3Array != null) {
// keep only first occurrence of .
// and the number of places specified by decimalPlaces or the entire string if decimalPlaces < 0
var reg3Right = temp.substring(reg3Array.index + reg3Array[0].length);
reg3Right = reg3Right.replace(reg3, '');
reg3Right = decimalPlaces > 0 ? reg3Right.substring(0, decimalPlaces) : reg3Right;
temp = temp.substring(0,reg3Array.index) + '.' + reg3Right;
}
}

obj.value = temp;
}
function blockNonNumbers(obj, e, allowDecimal, allowNegative)
{
var key;
var isCtrl = false;
var keychar;
var reg;

if(window.event) {
key = e.keyCode;
isCtrl = window.event.ctrlKey
}
else if(e.which) {
key = e.which;
isCtrl = e.ctrlKey;
}

if (isNaN(key)) return true;

keychar = String.fromCharCode(key);

// check for backspace or delete, or if Ctrl was pressed
if (key == 8 || isCtrl)
{
return true;
}
 
reg = /\d/;
var isFirstN = allowNegative ? keychar == '-' && obj.value.indexOf('-') == -1 : false;
var isFirstD = allowDecimal ? keychar == '.' && obj.value.indexOf('.') == -1 : false;

return isFirstN || isFirstD || reg.test(keychar);
}
// end of price text-box allow numeric and allow 2 decimal points only
</script>
 

<input type="text" name="price" value="" size="10" onblur="extractNumber(this,2,false);" onkeyup="extractNumber(this,2,false);" onkeypress="return blockNonNumbers(this, event, true, false);" />

There is a control called RegularExpressionValidator available with ASP.NET. Use that to validate the TextBox. Make sure you set the ControlToValidate and the ValidationExpression for it.
 
This will validate on both the client side and the server side.
 
For more information about RegularExpressionValidator, try the following link:
 
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.regularexpressionvalidator(v=vs.90).aspx[^]
 
If you wish to validate only via JavaScript, see the following link:
 
http://www.devarticles.com/c/a/JavaScript/Form-Validation-with-JavaScript-Regular-Expressions-Part-1/[^]

And the issue? You should have tried at least once.
 
Not just that, see how many search results are there for the keyword: enter decimal number into textbox.[^]
 

First link: Allow only numbers and dot in script[^]
 
Further, there are free regex tools online /stand alone that you can use to write correct regex.

Try this
 

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server"
                    ControlToValidate="TextBox1" ErrorMessage=" Enter Decimal Values Only"
                    ValidationExpression="/^(?:\d*\.\d{1,2}|\d+ $/"></asp:RegularExpressionValidator>

<asp:textbox id="TextBox1" runat="server" style="z-index: 100; left: 259px; position: absolute;<br mode=" hold=" />top: 283px" validationgroup="check" xmlns:asp="#unknown"></asp:textbox>
 
<asp:regularexpressionvalidator id="RegularExpressionValidator1" runat="server" controltovalidate="TextBox1" xmlns:asp="#unknown">
ErrorMessage="Please Enter Only Numbers" Style="z-index: 101; left: 424px; position: absolute;
top: 285px" ValidationExpression="^\d+$" ValidationGroup="check"></asp:regularexpressionvalidator>

i think this code is helpfull to u...
 
Regard
Sham

Use FilteredTextBoxExtender, which is available through AJAX Control Toolkit.
Check this link for more details :-
 
http://ajaxcontroltoolkit.codeplex.com/SourceControl/changeset/view/1014bf767f65#SampleWebSites%2fAjaxControlToolkitSampleSite%2fFilteredTextBox%2fFilteredTextBox.aspx[^]

u should use javascript function :
 

function fnAllowDigitsOnly(key)
{
 
     
        var keycode = (key.which) ? key.which : key.keyCode;
        if((keycode<48||keycode>57)&&(keycode!=8))
         {
            return false;
         }
}
 

// .cs code on page load 

if(!IsPostBack)
{
 txtextension.Attributes.Add("onkeypress", "return fnAllowDigitsOnly(event)");
}
 

hi
 
u try this
 

Private Sub txtinput_TextChanged(sender As Object, e As System.EventArgs) Handles txtinput.TextChanged
 

 
       If (txtinput.Text) Like ("*[!0-9,.]*") Then
 
           txtinput.Text = ""
           MessageBox.Show("enter string value")
       End If
 

   End Sub

only number and . and , are allowed
if u no need , and . means
write coding as

If (txtinput.Text) Like ("*[!0-9]*") Then

 
i think this will help u
 
happy coding...

refere these both link
 

 
http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/FilteredTextBox/FilteredTextBox.aspx[^]
 
and
A Custom TextBox with Regular Expression Validation Using MC++[^]