i have used the following code to validate the username it is working fine

=============================================
if( $username == "" ||  !preg_match("/^[a-z0-9]+(?:_[a-z0-9]+)?$/i", $username) )
{
$error.="User name cannot be blank or has special characters";
}     
=============================================

it does not accept UNDERSCORE at the beginning or end however while i was testing with different special characters except for # the validation works fine for all other special characters.

for example if i enter the user name as = abc#123

in this case # sign and what comes after # sign is being ignored. so in this case the username is being read as abc ONLY and not abc#123

this is very strange, how can i still validate # sign and tell the user that # sign is not a valid username like i have been doing with any other special characters like = !@$...........

please advice.

thanks.

I have tried the same code as you have given, its working fine.
 
I have uploaded below code at "http://pankajk.110mb.com/valid.php" you can check.


<?php
if(isset($_POST) && trim($_POST['name'])!='' ){
    $username = $_POST['name'];
    if( $username == "" ||  !preg_match("/^[a-z0-9]+(?:_[a-z0-9]+)?$/i", $username) ){
        echo "User name cannot be blank or has special characters";
    } else {
        echo "Valid";
    }
}
?>
<form action="" method="post">
<input type="text" name="name" />
</form>

thanks for replying. i have looked at the process and realised that i am using GET method.

following is the page sequence.

first page = register.php here a user enters a username and clicks on an image to find out
if the username is available or not. using a javascript function of onclick i am reading the

value entered in the form in javascript as
=============================================
var useri = document.registrationform.username
var valueofuseri = document.registrationform.username.value

var recui = /^\s{1,}$/g;

if ((useri.value==null) || (useri.value=="") || (useri.length=="") ||

(useri.value.search(recui))> -1)
{
alert("Please Enter a User Name")
return false
}

window.open("checkusernamei.php?theusernameis="+valueofuseri, "titleforavailabilityi",
"width=680,  height=275, status=1, scrollbars=1, resizeable=yes");

============================================

second page = checkusernamei.php = this file uses GET to read what was entered in the form.
============================================
$username = $_GET["theusernameis"];

if( $username == "" ||  !preg_match("/^[a-z0-9]+(?:_[a-z0-9]+)?$/i", $username) )
{
echo "username is blank or has special characters";
}
============================================
the # sign is being ignored only if the image is clicked in order to check the username, if
the user enters abc#123 and clicks the submit button without clicking on the checkuser image button then my php validation for username shows an error message.

===========================================================
if( $username == "" ||  !preg_match("/^[a-z0-9]+(?:_[a-z0-9]+)?$/i", $username) )
{ echo "display error message for username"; }
===========================================================
now the problem is with clicking the image only and passing value using GET method how can i fix this problem.

please advice.

thanks.