PHP - Validating # sign in php

my question is about validation using php. i am validating a username which a user would

enter and clicks on a image to find if that username is available. example if a user enters

abc#123 php file is reading this value as abc ONLY which i do not want instead the php file

should read as abc#123. following is the sequence of pages. please advice the solution.

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");

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

i have used a alert message in javascript to display the value, javascript is able to

capture all the characters entered which is abc#123

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 the value to checkusernamei.php

using GET method
i have also used an echo statement in checkusernamei.php as
echo "value of username is ". $username; = this displays abc and not abc#123

how can i fix this problem wherein checkusernamei.php will be able to read abc#123. also in

this checkusernamei.php file i have a select query which will read if the username already

exists in the table. presently as checkusernamei.php is reading abc ONLY the select query is

also passing abc and not abc#123

$select = "Select username from table where username = '$username'";

please advice.

thanks.