javascript form validation

validateUsername ( )The function below checks if the user entered anything at all in the username field. If it’s not blank, we check the length of the string and permit only usernames that are between 5 and 15 characters. Next, we use the JavaScript regular expression /\W/ to forbid illegal characters from appearing in usernames. We want to allow only letters, numbers and underscopes.

function validateUsername(fld) {
var error = “”;
var illegalChars = /\W/; // allow letters, numbers, and underscores

if (fld.value == “”) {
fld.style.background = ‘Yellow’;
error = “You didn’t enter a username.\n”;
} else if ((fld.value.length < 5) || (fld.value.length > 15)) {
fld.style.background = ‘Yellow’;
error = “The username is the wrong length.\n”;
} else if (illegalChars.test(fld.value)) {
fld.style.background = ‘Yellow’;
error = “The username contains illegal characters.\n”;
} else {
fld.style.background = ‘White’;
}
return error;
}

validatePassword ( )The function below checks the password field for blankness and allow only letters and numbers – no underscopes this time. So we should use a new regular expression to forbid underscopes. This one /[\W_]/ allow only letters and numbers. Next, we want to permit only passwords that contain letters and at least one numeral. For that we use the seacrh() method and two more regular expressions: /(a-z)+/ and /(0-9)/.

function validatePassword(fld) {
var error = “”;
var illegalChars = /[\W_]/; // allow only letters and numbers

if (fld.value == “”) {
fld.style.background = ‘Yellow’;
error = “You didn’t enter a password.\n”;
} else if ((fld.value.length < 7) || (fld.value.length > 15)) {
error = “The password is the wrong length. \n”;
fld.style.background = ‘Yellow’;
} else if (illegalChars.test(fld.value)) {
error = “The password contains illegal characters.\n”;
fld.style.background = ‘Yellow’;
} else if (!((fld.value.search(/(a-z)+/)) && (fld.value.search(/(0-9)+/)))) {
error = “The password must contain at least one numeral.\n”;
fld.style.background = ‘Yellow’;
} else {
fld.style.background = ‘White’;
}
return error;
}

validateEmail ( )Next we want to see if the email address the user entered is real. This means that the input data must contain at least an @ sign and a dot (.). Also, the @ must not be the first character of the email address, and the last dot must at least be one character after the @ sign.At first we check if the user entered anything at all in the email field. Next, we use regular expression and the test() method to check the field for a compliance. Also we will use trim() function that will trim leading whitespace off the string. This won’t be perfect validation — it is possible to slip not compliant addresses by it — but it’s normally good enough.

validatePhone ( )The function below checks if the phone number is valid. At first we use regular expression and the replace() method to clear out any spacer characters. Next, we use the isNaN() function to check if the phone number contain only numbers. At last we check the length of the string and permit only phone numbers with 10 digits.

function validatePhone(fld) {
var error = “”;
var stripped = fld.value.replace(/[\(\)\.\-\ ]/g, ”);

if (fld.value == “”) {
error = “You didn’t enter a phone number.\n”;
fld.style.background = ‘Yellow’;
} else if (isNaN(parseInt(stripped))) {
error = “The phone number contains illegal characters.\n”;
fld.style.background = ‘Yellow’;
} else if (!(stripped.length == 10)) {
error = “The phone number is the wrong length. Make sure you included an area code.\n”;
fld.style.background = ‘Yellow’;
}
return error;
}

source: http://www.webcheatsheet.com/javascript/form_validation.php