function email_validate(){
var email=document.getElementById('email').value
var regMail = /^([_a-zA-Z0-9-]+)(.[_a-zA-Z0-9-]+)*@([a-zA-Z0-9-]+.)+([a-zA-Z]{2,3})$/;

if(regMail.test(email) == false){
alert("Email address is not valid yet.");
return false;
}
}

Add this to your head section:
##################################################################################
function echeck(str)
{
var at="@"
var dot="."
var lat=str.indexOf(at)
var lstr=str.length
var ldot=str.indexOf(dot)

if (str.indexOf(at)==-1){
alert("Invalid E-mail ID")
return false
}

if (str.indexOf(at)==-1 || str.indexOf(at)==0 || str.indexOf(at)==lstr){
alert("Invalid E-mail ID")
return false
}

if (str.indexOf(dot)==-1 || str.indexOf(dot)==0 || str.indexOf(dot)==lstr){
alert("Invalid E-mail ID")
return false
}

if (str.indexOf(at,(lat+1))!=-1){
alert("Invalid E-mail ID")
return false
}

if (str.substring(lat-1,lat)==dot || str.substring(lat+1,lat+2)==dot){
alert("Invalid E-mail ID")
return false
}

if (str.indexOf(dot,(lat+2))==-1){
alert("Invalid E-mail ID")
return false
}

if (str.indexOf(" ")!=-1){
alert("Invalid E-mail ID")
return false
}
return true
}

function ValidateForm(){
var emailID=document.frmSample.txtEmail

if ((emailID.value==null)||(emailID.value=="")){
alert("Please Enter your Email ID")
emailID.focus()
return false
}

if (echeck(emailID.value)==false){
emailID.value=""
emailID.focus()
return false
}
return true
}
####################################################################################
<form name="frmSample" method="post" action="#" onSubmit="return ValidateForm()">
<p>Enter an Email Address :
<input type="text" name="txtEmail">
</p>
<p>
<input type="submit" name="Submit" value="Submit">
</p>
</form>

<script type="text/javascript">
function validate(form) {
var total=""
for(var i=0; i < document.form1.scripts.length; i++){
if(document.form1.scripts[i].checked)
total +=document.form1.scripts[i].value + "
"
}
if(total=="")
alert("select scripts") else
alert (total)

return false;
} </script>

<table border="0" width="50%" cellspacing="0" cellpadding="0" ><form name=form1 method=post action=action_page.php onsubmit="return validate(this)"><input type=hidden name=todo value=post>
<tr>
<td align=center bgcolor="#f1f1f1">
<font face="verdana" size="2">
<b>Scripts You know</b>

<input type=checkbox
name=scripts
value="JavaScript">JavaScript

<input type=checkbox
name=scripts
value="PHP">PHP

<input type=checkbox
name=scripts
value="HTML">HTML
</td>
</tr>

<tr bgcolor="#ffffff"><td align=center ><input type=submit value=Submit> <input type=reset value=Reset></td></tr>

</table></form>

<script type="text/javascript">
function validate(form) {
// Checking if at least one period button is selected. Or not.
if (!document.form1.sex[0].checked && !document.form1.sex[1].checked){

alert("Please Select Sex"); return false;}
</script>

<table border=``0`` width=``50%`` cellspacing=``0`` cellpadding=``0`` ><form name=form1 method=post action=action_page.php onsubmit=``return validate(this)``><input type=hidden name=todo value=post>

<tr bgcolor=``#ffffff``><td align=center ><font face=``verdana`` size=``2``><b>Sex</b><input type=radio name=sex value=``male``>Male </font><input type=radio name=sex value=``female``><font face=``verdana`` size=``2``>Female</font></td></tr>

<tr bgcolor=``#ffffff``><td align=center ><input type=submit value=Submit> <input type=reset value=Reset></td></tr>

</table></form>

<script language = "Javascript">
var digits = "0123456789";
var phoneNumberDelimiters = "()- ";
var validWorldPhoneChars = phoneNumberDelimiters + "+";
var minDigitsInIPhoneNumber = 10;

function isInteger(s)
{ var i;
for (i = 0; i < s.length; i++)
{
// Check that current character is number.
var c = s.charAt(i);
if (((c < "0") || (c > "9"))) return false;
}
// All characters are numbers.
return true;
}
function trim(s)
{ var i;
var returnString = "";
// Search through string````````````````s characters one by one.
// If character is not a whitespace, append to returnString.
for (i = 0; i < s.length; i++)
{
// Check that current character isn````````````````t whitespace.
var c = s.charAt(i);
if (c != " ") returnString += c;
}
return returnString;
}
function stripCharsInBag(s, bag)
{ var i;
var returnString = "";
// Search through string````````````````s characters one by one.
// If character is not in bag, append to returnString.
for (i = 0; i < s.length; i++)
{
// Check that current character isn````````````````t whitespace.
var c = s.charAt(i);
if (bag.indexOf(c) == -1) returnString += c;
}
return returnString;
}

function checkInternationalPhone(strPhone){
var bracket=3
strPhone=trim(strPhone)
if(strPhone.indexOf("+")>1) return false
if(strPhone.indexOf("-")!=-1)bracket=bracket+1
if(strPhone.indexOf("(")!=-1 && strPhone.indexOf("(")>bracket)return false
var brchr=strPhone.indexOf("(")
if(strPhone.indexOf("(")!=-1 && strPhone.charAt(brchr+2)!=")")return false
if(strPhone.indexOf("(")==-1 && strPhone.indexOf(")")!=-1)return false
s=stripCharsInBag(strPhone,validWorldPhoneChars);
return (isInteger(s) && s.length >= minDigitsInIPhoneNumber);
}

function ValidateForm(){
var Phone=document.frmSample.txtPhone

if ((Phone.value==null)||(Phone.value=="")){
alert("Please Enter your Phone Number")
Phone.focus()
return false
}
if (checkInternationalPhone(Phone.value)==false){
alert("Please Enter a Valid Phone Number")
Phone.value=""
Phone.focus()
return false
}
return true
}
</script>



<form name="frmSample" method="post" action="#" onSubmit="return ValidateForm()">
<p>Enter a phone number :
<input type="text" name="txtPhone">
</p>
<p>
<input type="submit" name="Submit" value="Submit">
</p>
</form>

/*============================================================================*/

/*

This routine checks the credit card number. The following checks are made:

1. A number has been provided
2. The number is a right length for the card
3. The number has an appropriate prefix for the card
4. The number has a valid modulus 10 number check digit if required

If the validation fails an error is reported.

The structure of credit card formats was gleaned from a variety of sources on
the web, although the best is probably on Wikepedia ("Credit card number"):

http://en.wikipedia.org/wiki/Credit_card_number

Parameters:
cardnumber number on the card
cardname name of card as defined in the card list below

Author: John Gardner
Date: 1st November 2003
Updated: 26th Feb. 2005 Additional cards added by request
Updated: 27th Nov. 2006 Additional cards added from Wikipedia
Updated: 18th Jan. 2008 Additional cards added from Wikipedia
Updated: 26th Nov. 2008 Maestro cards extended
Updated: 19th Jun. 2009 Laser cards extended from Wikipedia

*/

/*
If a credit card number is invalid, an error reason is loaded into the
global ccErrorNo variable. This can be be used to index into the global error
string array to report the reason to the user if required:

e.g. if (!checkCreditCard (number, name) alert (ccErrors(ccErrorNo);
*/

var ccErrorNo = 0;
var ccErrors = new Array ()

ccErrors [0] = "Unknown card type";
ccErrors [1] = "No card number provided";
ccErrors [2] = "Credit card number is in invalid format";
ccErrors [3] = "Credit card number is invalid";
ccErrors [4] = "Credit card number has an inappropriate number of digits";

function checkCreditCard (cardnumber, cardname) {

// Array to hold the permitted card characteristics
var cards = new Array();

// Define the cards we support. You may add addtional card types.

// Name: As in the selection box of the form - must be same as user''s
// Length: List of possible valid lengths of the card number for the card
// prefixes: List of possible prefixes for the card
// checkdigit Boolean to say whether there is a check digit

cards [0] = {name: "Visa",
length: "13,16",
prefixes: "4",
checkdigit: true};
cards [1] = {name: "MasterCard",
length: "16",
prefixes: "51,52,53,54,55",
checkdigit: true};
cards [2] = {name: "DinersClub",
length: "14,16",
prefixes: "305, 36, 38, 54,55",
checkdigit: true};
cards [3] = {name: "CarteBlanche",
length: "14",
prefixes: "300,301,302,303,304,305",
checkdigit: true};
cards [4] = {name: "AmEx",
length: "15",
prefixes: "34,37",
checkdigit: true};
cards [5] = {name: "Discover",
length: "16",
prefixes: "6011,622,64,65",
checkdigit: true};
cards [6] = {name: "JCB",
length: "16",
prefixes: "35",
checkdigit: true};
cards [7] = {name: "enRoute",
length: "15",
prefixes: "2014,2149",
checkdigit: true};
cards [8] = {name: "Solo",
length: "16,18,19",
prefixes: "6334, 6767",
checkdigit: true};
cards [9] = {name: "Switch",
length: "16,18,19",
prefixes: "4903,4905,4911,4936,564182,633110,6333,6759",
checkdigit: true};
cards [10] = {name: "Maestro",
length: "12,13,14,15,16,18,19",
prefixes: "5018,5020,5038,6304,6759,6761",
checkdigit: true};
cards [11] = {name: "VisaElectron",
length: "16",
prefixes: "417500,4917,4913,4508,4844",
checkdigit: true};
cards [12] = {name: "LaserCard",
length: "16,17,18,19",
prefixes: "6304,6706,6771,6709",
checkdigit: true};

// Establish card type
var cardType = -1;
for (var i=0; i
// See if it is this card (ignoring the case of the string)
if (cardname.toLowerCase () == cards[i].name.toLowerCase()) {
cardType = i;
break;
}
}

// If card type not found, report an error
if (cardType == -1) {
ccErrorNo = 0;
return false;
}

// Ensure that the user has provided a credit card number
if (cardnumber.length == 0) {
ccErrorNo = 1;
return false;
}

// Now remove any spaces from the credit card number
cardnumber = cardnumber.replace (/\s/g, "");

// Check that the number is numeric
var cardNo = cardnumber
var cardexp = /^[0-9]{13,19}$/;
if (!cardexp.exec(cardNo)) {
ccErrorNo = 2;
return false;
}

// Now check the modulus 10 check digit - if required
if (cards[cardType].checkdigit) {
var checksum = 0; // running checksum total
var mychar = ""; // next char to process
var j = 1; // takes value of 1 or 2

// Process each digit one by one starting at the right
var calc;
for (i = cardNo.length - 1; i >= 0; i--) {

// Extract the next digit and multiply by 1 or 2 on alternative digits.
calc = Number(cardNo.charAt(i)) * j;

// If the result is in two digits add 1 to the checksum total
if (calc > 9) {
checksum = checksum + 1;
calc = calc - 10;
}

// Add the units element to the checksum total
checksum = checksum + calc;

// Switch the value of j
if (j ==1) {j = 2} else {j = 1};
}

// All done - if checksum is divisible by 10, it is a valid modulus 10.
// If not, report an error.
if (checksum % 10 != 0) {
ccErrorNo = 3;
return false;
}
}

// The following are the card-specific checks we undertake.
var LengthValid = false;
var PrefixValid = false;
var undefined;

// We use these for holding the valid lengths and prefixes of a card type
var prefix = new Array ();
var lengths = new Array ();

// Load an array with the valid prefixes for this card
prefix = cards[cardType].prefixes.split(",");

// Now see if any of them match what we have in the card number
for (i=0; i var exp = new RegExp ("^" + prefix[i]);
if (exp.test (cardNo)) PrefixValid = true;
}

// If it isn''t a valid prefix there''s no point at looking at the length
if (!PrefixValid) {
ccErrorNo = 3;
return false;
}

// See if the length is valid for this card
lengths = cards[cardType].length.split(",");
for (j=0; j if (cardNo.length == lengths[j]) LengthValid = true;
}

// See if all is OK by seeing if the length was valid. We only check the
// length if all else was hunky dory.
if (!LengthValid) {
ccErrorNo = 4;
return false;
};

// The credit card is in the required format.
return true;
}

/*============================================================================*/

function getCookie( name ) {
var start = document.cookie.indexOf( name + "=" );
var len = start + name.length + 1;
if ( ( !start ) && ( name != document.cookie.substring( 0, name.length ) ) ) {
return null;
}
if ( start == -1 ) return null;
var end = document.cookie.indexOf( '';'', len );
if ( end == -1 ) end = document.cookie.length;
return unescape( document.cookie.substring( len, end ) );
}

function setCookie( name, value, expires, path, domain, secure ) {
var today = new Date();
today.setTime( today.getTime() );
if ( expires ) {
expires = expires * 1000 * 60 * 60 * 24;
}
var expires_date = new Date( today.getTime() + (expires) );
document.cookie = name+''=''+escape( value ) +
( ( expires ) ? '';expires=''+expires_date.toGMTString() : '''' ) + //expires.toGMTString()
( ( path ) ? '';path='' + path : '''' ) +
( ( domain ) ? '';domain='' + domain : '''' ) +
( ( secure ) ? '';secure'' : '''' );
}

function deleteCookie( name, path, domain ) {
if ( getCookie( name ) ) document.cookie = name + ''='' +
( ( path ) ? '';path='' + path : '''') +
( ( domain ) ? '';domain='' + domain : '''' ) +
'';expires=Thu, 01-Jan-1970 00:00:01 GMT'';
}

function getElementsByClass(searchClass,node,tag) {
var classElements = new Array();
if ( node == null )
node = document;
if ( tag == null )
tag = ''*'';
var els = node.getElementsByTagName(tag);
var elsLen = els.length;
var pattern = new RegExp(''(^|\\\\s)''+searchClass+''(\\\\s|$)'');
for (i = 0, j = 0; i < elsLen; i++) {
if ( pattern.test(els[i].className) ) {
classElements[j] = els[i];
j++;
}
}
return classElements;
}

function addEvent(elm, evType, fn, useCapture) {
if (elm.addEventListener) {
elm.addEventListener(evType, fn, useCapture);
return true;
}
else if (elm.attachEvent) {
var r = elm.attachEvent(''on'' + evType, fn);
return r;
}
else {
elm[''on'' + evType] = fn;
}
}

function addLoadEvent(func) {
var oldonload = window.onload;
if (typeof window.onload != ''function'') {
window.onload = func;
}
else {
window.onload = function() {
oldonload();
func();
}
}
}

function toggle(obj) {
var el = document.getElementById(obj);
if ( el.style.display != ''none'' ) {
el.style.display = ''none'';
}
else {
el.style.display = '''';
}
}

function insertAfter(parent, node, referenceNode) {
parent.insertBefore(node, referenceNode.nextSibling);
}