javascript for validating textbox

1 ) Setting the Text box to take only 12 Values.

Why dont you limit the text for size modifying <input> tage used for the text box ?

<input type="text" size="value" maxlength="12"> </input>

2) First 2 should be BA.

You can use Substring operator and string comparison to achieve this.
Initialise actualString with value BA.

if ( stringSource.substring(0,1).toString() != actualString.toString() )
  {
      // Display Error
  }

3) Check the rest of the 10 digits are number.

if ( isNaN(stringSource.substring(2,11)))
  {
     // Display Erro
  }

You can add the above contents into one function and then set it to work for the textchanged event.

BR//
Harsha</input>

Use regular expression to check. It's simple
here's a function you can use

function Check()
{
 var ex = /^BA[0-9]{10}$/;
 if(ex.test(document.getElementById('YourTextBoxId').value) == false)
 {
    // your code to warn the user of incorrect input
 }
}

You can call this function when the user leaves the textbox. Use the onblur event for it

<input type="text" onblur="Check();" />

Note that i have assumed that all 10 integers have to be entered. If you want it to be optional, replace the {10} above with {0,10}

function validate()
   {
       var tb=document.getElementById ('TextBox1').value
       var f=tb.charAt(0)
       var s=tb.charAt(1)
       if(tb.value=="" )
       {
            alert('enter value')

       }
       if(f!='B')
       {
         alert ('First Char Should be B')
       }
       if(s!='A')
       {
          alert ('Second Char Should be A')
       }
      for(var i=3;i<13;i++)
      {
       if((tb.charAt(i) >= 48 && tb.charAt(i) <= 57) || tb.charAt(i) == 8 || (tb.charAt(i) >= 96 && tb.charAt(i) <= 105))
       {
                alert ('Enter Digits only')
        }

       }
       if(tb.length>13)
        {
          alert('Input Value Exceeds morethan 12')
          tb.value="";
         }



    return true

   }
</script>

I think u will not get this readymade validation code...!
You need to check that textbox value in a function

function CheckValue(src)
{
  if(src..length == 12)
  {
   if(src...2 = ='BA')
   {
      if(src....from 3 to 10 ==numbers)
        {
           return true;
        }
      else
        {
           return false;
        }    
   }
   else
   {
     return false;
   }
  }
  else
   {
     return false;
   }
}

I dont know the exact functions in javascript therefore i put ..... insted of function.
but i think this logic will work surely..!
All the Best !! Raj,