Develop Reference

How do i restrict some specific special character from input? [duplicate]

How do I block special characters from being typed into an input field with jquery?

A simple example using a regular expression which you could change to allow/disallow whatever you like.
$('input').on('keypress', function (event) {
var regex = new RegExp("^[a-zA-Z0-9]+$");
var key = String.fromCharCode(!event.charCode ? event.which : event.charCode);
if (!regex.test(key)) {
event.preventDefault();
return false;
}
});

I was looking for an answer that restricted input to only alphanumeric characters, but still allowed for the use of control characters (e.g., backspace, delete, tab) and copy+paste. None of the provided answers that I tried satisfied all of these requirements, so I came up with the following using the input event.
$('input').on('input', function() {
$(this).val($(this).val().replace(/[^a-z0-9]/gi, ''));
});
Edit:
As rinogo pointed out in the comments, the above code snippet forces the cursor to the end of the input when typing in the middle of the input text. I believe the code snippet below solves this problem.
$('input').on('input', function() {
var c = this.selectionStart,
r = /[^a-z0-9]/gi,
v = $(this).val();
if(r.test(v)) {
$(this).val(v.replace(r, ''));
c--;
}
this.setSelectionRange(c, c);
});

Short answer: prevent the 'keypress' event:
$("input").keypress(function(e){
var charCode = !e.charCode ? e.which : e.charCode;
if(/* Test for special character */ )
e.preventDefault();
})
Long answer: Use a plugin like jquery.alphanum
There are several things to consider when picking a solution:
Pasted text
Control characters like backspace or F5 may be prevented by the above code.
é, í, ä etc
Arabic or Chinese...
Cross Browser compatibility
I think this area is complex enough to warrant using a 3rd party plugin. I tried out several of the available plugins but found some problems with each of them so I went ahead and wrote jquery.alphanum. The code looks like this:
$("input").alphanum();
Or for more fine-grained control, add some settings:
$("#username").alphanum({
allow : "€$£",
disallow : "xyz",
allowUpper : false
});
Hope it helps.

Use simple onkeypress event inline.
<input type="text" name="count" onkeypress="return /[0-9a-zA-Z]/i.test(event.key)">

Use HTML5's pattern input attribute!
<input type="text" pattern="^[a-zA-Z0-9]+$" />

Use regex to allow/disallow anything. Also, for a slightly more robust version than the accepted answer, allowing characters that don't have a key value associated with them (backspace, tab, arrow keys, delete, etc.) can be done by first passing through the keypress event and check the key based on keycode instead of value.
$('#input').bind('keydown', function (event) {
switch (event.keyCode) {
case 8: // Backspace
case 9: // Tab
case 13: // Enter
case 37: // Left
case 38: // Up
case 39: // Right
case 40: // Down
break;
default:
var regex = new RegExp("^[a-zA-Z0-9.,/ $#()]+$");
var key = event.key;
if (!regex.test(key)) {
event.preventDefault();
return false;
}
break;
}
});

Your textbox:
<input type="text" id="name">
Your javascript:
$("#name").keypress(function(event) {
var character = String.fromCharCode(event.keyCode);
return isValid(character);
});
function isValid(str) {
return !/[~`!##$%\^&*()+=\-\[\]\\';,/{}|\\":<>\?]/g.test(str);
}

Take a look at the jQuery alphanumeric plugin. https://github.com/KevinSheedy/jquery.alphanum
//All of these are from their demo page
//only numbers and alpha characters
$('.sample1').alphanumeric();
//only numeric
$('.sample4').numeric();
//only numeric and the .
$('.sample5').numeric({allow:"."});
//all alphanumeric except the . 1 and a
$('.sample6').alphanumeric({ichars:'.1a'});

this is an example that prevent the user from typing the character "a"
$(function() {
$('input:text').keydown(function(e) {
if(e.keyCode==65)
return false;
});
});
key codes refrence here:
http://www.expandinghead.net/keycode.html

I use this code modifying others that I saw. Only grand to the user write if the key pressed or pasted text pass the pattern test (match) (this example is a text input that only allows 8 digits)
$("input").on("keypress paste", function(e){
var c = this.selectionStart, v = $(this).val();
if (e.type == "keypress")
var key = String.fromCharCode(!e.charCode ? e.which : e.charCode)
else
var key = e.originalEvent.clipboardData.getData('Text')
var val = v.substr(0, c) + key + v.substr(c, v.length)
if (!val.match(/\d{0,8}/) || val.match(/\d{0,8}/).toString() != val) {
e.preventDefault()
return false
}
})

$(function(){
$('input').keyup(function(){
var input_val = $(this).val();
var inputRGEX = /^[a-zA-Z0-9]*$/;
var inputResult = inputRGEX.test(input_val);
if(!(inputResult))
{
this.value = this.value.replace(/[^a-z0-9\s]/gi, '');
}
});
});

Write some javascript code on onkeypress event of textbox.
as per requirement allow and restrict character in your textbox
function isNumberKeyWithStar(evt) {
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57) && charCode != 42)
return false;
return true;
}
function isNumberKey(evt) {
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57))
return false;
return true;
}
function isNumberKeyForAmount(evt) {
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57) && charCode != 46)
return false;
return true;
}

To replace special characters, space and convert to lower case
$(document).ready(function (){
$(document).on("keyup", "#Id", function () {
$("#Id").val($("#Id").val().replace(/[^a-z0-9\s]/gi, '').replace(/[_\s]/g, '').toLowerCase());
});
});

Yes you can do by using jQuery as:
<script>
$(document).ready(function()
{
$("#username").blur(function()
{
//remove all the class add the messagebox classes and start fading
$("#msgbox").removeClass().addClass('messagebox').text('Checking...').fadeIn("slow");
//check the username exists or not from ajax
$.post("user_availability.php",{ user_name:$(this).val() } ,function(data)
{
if(data=='empty') // if username is empty
{
$("#msgbox").fadeTo(200,0.1,function() //start fading the messagebox
{
//add message and change the class of the box and start fading
$(this).html('Empty user id is not allowed').addClass('messageboxerror').fadeTo(900,1);
});
}
else if(data=='invalid') // if special characters used in username
{
$("#msgbox").fadeTo(200,0.1,function() //start fading the messagebox
{
//add message and change the class of the box and start fading
$(this).html('Sorry, only letters (a-z), numbers (0-9), and periods (.) are allowed.').addClass('messageboxerror').fadeTo(900,1);
});
}
else if(data=='no') // if username not avaiable
{
$("#msgbox").fadeTo(200,0.1,function() //start fading the messagebox
{
//add message and change the class of the box and start fading
$(this).html('User id already exists').addClass('messageboxerror').fadeTo(900,1);
});
}
else
{
$("#msgbox").fadeTo(200,0.1,function() //start fading the messagebox
{
//add message and change the class of the box and start fading
$(this).html('User id available to register').addClass('messageboxok').fadeTo(900,1);
});
}
});
});
});
</script>
<input type="text" id="username" name="username"/><span id="msgbox" style="display:none"></span>
and script for your user_availability.php will be:
<?php
include'includes/config.php';
//value got from the get method
$user_name = trim($_POST['user_name']);
if($user_name == ''){
echo "empty";
}elseif(preg_match('/[\'^£$%&*()}{##~?><>,|=_+¬-]/', $user_name)){
echo "invalid";
}else{
$select = mysql_query("SELECT user_id FROM staff");
$i=0;
//this varible contains the array of existing users
while($fetch = mysql_fetch_array($select)){
$existing_users[$i] = $fetch['user_id'];
$i++;
}
//checking weather user exists or not in $existing_users array
if (in_array($user_name, $existing_users))
{
//user name is not availble
echo "no";
}
else
{
//user name is available
echo "yes";
}
}
?>
I tried to add for / and \ but not succeeded.
You can also do it by using javascript & code will be:
<!-- Check special characters in username start -->
<script language="javascript" type="text/javascript">
function check(e) {
var keynum
var keychar
var numcheck
// For Internet Explorer
if (window.event) {
keynum = e.keyCode;
}
// For Netscape/Firefox/Opera
else if (e.which) {
keynum = e.which;
}
keychar = String.fromCharCode(keynum);
//List of special characters you want to restrict
if (keychar == "'" || keychar == "`" || keychar =="!" || keychar =="#" || keychar =="#" || keychar =="$" || keychar =="%" || keychar =="^" || keychar =="&" || keychar =="*" || keychar =="(" || keychar ==")" || keychar =="-" || keychar =="_" || keychar =="+" || keychar =="=" || keychar =="/" || keychar =="~" || keychar =="<" || keychar ==">" || keychar =="," || keychar ==";" || keychar ==":" || keychar =="|" || keychar =="?" || keychar =="{" || keychar =="}" || keychar =="[" || keychar =="]" || keychar =="¬" || keychar =="£" || keychar =='"' || keychar =="\\") {
return false;
} else {
return true;
}
}
</script>
<!-- Check special characters in username end -->
<!-- in your form -->
User id : <input type="text" id="txtname" name="txtname" onkeypress="return check(event)"/>

just the numbers:
$('input.time').keydown(function(e) { if(e.keyCode>=48 &&
e.keyCode<=57) {
return true; } else {
return false; } });
or for time including ":"
$('input.time').keydown(function(e) { if(e.keyCode>=48 &&
e.keyCode<=58) {
return true; } else {
return false; } });
also including delete and backspace:
$('input.time').keydown(function(e) { if((e.keyCode>=46 &&
e.keyCode<=58) || e.keyCode==8) { return true; } else {
return false; } });
unfortuneatly not getting it to work on a iMAC

Wanted to comment on Alex's comment to Dale's answer. Not possible (first need how much "rep"? That wont happen very soon.. strange system.)
So as an answer:
Backspace can be added by adding \b to the regex definition like this: [a-zA-Z0-9\b].
Or you simply allow the whole Latin range, including more or less anything "non exotic" characters (also control chars like backspace): ^[\u0000-\u024F\u20AC]+$
Only real unicode char outside latin there is the euro sign (20ac), add whatever you may need else.
To also handle input entered via copy&paste, simply also bind to the "change" event and check the input there too - deleting it or striping it / giving an error message like "not supported characters"..
if (!regex.test($j(this).val())) {
alert('your input contained not supported characters');
$j(this).val('');
return false;
}

Restrict specials characters on keypress. Here's a test page for key codes: http://www.asquare.net/javascript/tests/KeyCode.html
var specialChars = [62,33,36,64,35,37,94,38,42,40,41];
some_element.bind("keypress", function(event) {
// prevent if in array
if($.inArray(event.which,specialChars) != -1) {
event.preventDefault();
}
});
In Angular, I needed a proper currency format in my textfield. My solution:
var angularApp = angular.module('Application', []);
...
// new angular directive
angularApp.directive('onlyNum', function() {
return function( scope, element, attrs) {
var specialChars = [62,33,36,64,35,37,94,38,42,40,41];
// prevent these special characters
element.bind("keypress", function(event) {
if($.inArray(event.which,specialChars) != -1) {
prevent( scope, event, attrs)
}
});
var allowableKeys = [8,9,37,39,46,48,49,50,51,52,53,54,55,56
,57,96,97,98,99,100,101,102,103,104,105,110,190];
element.bind("keydown", function(event) {
if($.inArray(event.which,allowableKeys) == -1) {
prevent( scope, event, attrs)
}
});
};
})
// scope.$apply makes angular aware of your changes
function prevent( scope, event, attrs) {
scope.$apply(function(){
scope.$eval(attrs.onlyNum);
event.preventDefault();
});
event.preventDefault();
}
In the html add the directive
<input only-num type="text" maxlength="10" id="amount" placeholder="$XXXX.XX"
autocomplete="off" ng-model="vm.amount" ng-change="vm.updateRequest()">
and in the corresponding angular controller I only allow there to be only 1 period, convert text to number and add number rounding on 'blur'
...
this.updateRequest = function() {
amount = $scope.amount;
if (amount != undefined) {
document.getElementById('spcf').onkeypress = function (e) {
// only allow one period in currency
if (e.keyCode === 46 && this.value.split('.').length === 2) {
return false;
}
}
// Remove "." When Last Character and round the number on blur
$("#amount").on("blur", function() {
if (this.value.charAt(this.value.length-1) == ".") {
this.value.replace(".","");
$("#amount").val(this.value);
}
var num = parseFloat(this.value);
// check for 'NaN' if its safe continue
if (!isNaN(num)) {
var num = (Math.round(parseFloat(this.value) * 100) / 100).toFixed(2);
$("#amount").val(num);
}
});
this.data.amountRequested = Math.round(parseFloat(amount) * 100) / 100;
}
...

You don't need jQuery for this action
You can achieve this using plain JavaScript, You can put this in the onKeyUp event.
Restrict - Special Characters
e.target.value = e.target.value.replace(/[^\w]|_/g, '').toLowerCase()
Accept - Number only
e.target.value = e.target.value.replace(/[^0-9]/g, '').toLowerCase()
Accept - Small Alphabet only
e.target.value = e.target.value.replace(/[^0-9]/g, '').toLowerCase()
I could write for some more scenarios but I have to maintain the specific answer.
Note It will work with jquery, react, angular, and so on.

$(this).val($(this).val().replace(/[^0-9\.]/g,''));
if( $(this).val().indexOf('.') == 0){
$(this).val("");
}
//this is the simplest way
indexof is used to validate if the input started with "."

[User below code to restrict special character also
$(h.txtAmount).keydown(function (event) {
if (event.shiftKey) {
event.preventDefault();
}
if (event.keyCode == 46 || event.keyCode == 8) {
}
else {
if (event.keyCode < 95) {
if (event.keyCode < 48 || event.keyCode > 57) {
event.preventDefault();
}
}
else {
if (event.keyCode < 96 || event.keyCode > 105) {
event.preventDefault();
}
}
}
});]

Allow only numbers in TextBox (Restrict Alphabets and Special Characters)
/*code: 48-57 Numbers
8 - Backspace,
35 - home key, 36 - End key
37-40: Arrow keys, 46 - Delete key*/
function restrictAlphabets(e){
var x=e.which||e.keycode;
if((x>=48 && x<=57) || x==8 ||
(x>=35 && x<=40)|| x==46)
return true;
else
return false;
}

/**
* Forbids special characters and decimals
* Allows numbers only
* */
const numbersOnly = (evt) => {
let charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode === 46 && charCode > 31 && (charCode < 48 || charCode > 57)) {
return false;
}
let inputResult = /^[0-9]*$/.test(evt.target.value);
if (!inputResult) {
evt.target.value = evt.target.value.replace(/[^a-z0-9\s]/gi, '');
}
return true;
}

In HTML:
<input type="text" (keypress)="omitSpecialChar($event)"/>
In JS:
omitSpecialChar(event) {
const keyPressed = String.fromCharCode(event.keyCode);
const verifyKeyPressed = /^[a-zA-Z\' \u00C0-\u00FF]*$/.test(keyPressed);
return verifyKeyPressed === true;
}
In this example it is possible to type accents.

$(document).ready(function() {
$('#Description').bind('input', function() {
var c = this.selectionStart,
r = /[^a-z0-9 .]/gi,
v = $(this).val();
if (r.test(v)) {
$(this).val(v.replace(r, ''));
c--;
}
this.setSelectionRange(c, c);
if (!(checkEmpty($("#Description").val()))) {
$("#Description").val("");
} //1Apr2022 code end
});
$('#Description').on('change', function() {
if (!(checkEmpty($("#Description").val()))) {
$("#Description").val("");
} //1Apr2022 code end
});
});
function checkEmpty(field) { //1Apr2022 new code
if (field == "" ||
field == null ||
field == "undefinied") {
return false;
} else if (/^\s*$/.test(field)) {
return false;
} else {
return true;
}
}

A more enhanced form would be
$('input[type=text]').on('input', function() {
var c = this.selectionStart,
r = /[^a-z ]/gi,
v = $(this).val();
if(r.test(v)) {
$(this).val(v.replace(r, ''));
c--;
}
this.setSelectionRange(c, c);
});
Because it will allow you to enter space as well and it will only target the input fields with type text and wont bother the other input fields like email, password etc as normally we need special characters in email and password field

Detecting when CAPS LOCK is ON

I have this code for adding a keypress to a password. If capslock is on it will trigger. I got it from How do you tell if caps lock is on using JavaScript?
$("input[type='password']").keypress(function(e) {
var $warn = $(this).next(".capsWarn"); // handle the warning mssg
var kc = e.which; //get keycode
var isUp = (kc >= 65 && kc <= 90) ? true : false; // uppercase
var isLow = (kc >= 97 && kc <= 122) ? true : false; // lowercase
// event.shiftKey does not seem to be normalized by jQuery(?) for IE8-
var isShift = ( e.shiftKey ) ? e.shiftKey : ( (kc == 16) ? true : false ); // shift is pressed
// uppercase w/out shift or lowercase with shift == caps lock
if ( (isUp && !isShift) || (isLow && isShift) ) {
$warn.show();
} else {
$warn.hide();
}
}).after(capLock());
function capLock(e){
alert('CAPSLOCK is ON');
}
The original code has the message in a span:
...}).after(<span class='capsWarn error' style='display:none;'>CAPSLOCK is ON</span>);
I wanted it to be an alert message but it is not performing as I expect it to. On load it should alert the message but even if the capslock is on it is not showing the alert.
How do I get it to detect the key and present the alert?

$("input[type='password']").keypress(function(e) {
var $warn = $(this).next(".capsWarn");//can be removed since you are just using alert
var kc = e.which; //get keycode
var isUp = (kc >= 65 && kc <= 90) ? true : false; // uppercase
var isLow = (kc >= 97 && kc <= 122) ? true : false; // lowercase
// event.shiftKey does not seem to be normalized by jQuery(?) for IE8-
var isShift = ( e.shiftKey ) ? e.shiftKey : ( (kc == 16) ? true : false ); // shift is pressed
// uppercase w/out shift or lowercase with shift == caps lock
if ( (isUp && !isShift) || (isLow && isShift) ) {
capLock(); // alerts "CAPSLOCK is ON"
}
});
function capLock() {
alert('CAPSLOCK is ON');
}

Restriciting enterting of alphabets in jquery mobile

I am using a text box with type number. For xperia mobile's if i am using the same text box,it is showing a keyboard with all the value.So i am able to type any value. I want to restrict the user from entering any value.User have to enter only number.I tried the following code.
*
document.getElementById('loginPin').addEventListener('keyup', function(e){
var keyCode = ('which' in e) ? e.which : e.keyCode;
var isNumeric = (keyCode < 48 KeyboardEvent.DOM_VK_0 && keyCode > 57 KeyboardEvent.DOM_VK_9 ) ||
(keyCode < 96 KeyboardEvent.DOM_VK_NUMPAD0 && keyCode > 105 KeyboardEvent.DOM_VK_NUMPAD9 );
var modifiers = (event.altKey || event.ctrlKey || event.shiftKey);
return !isNumeric || modifiers;
var V = $(this).val();
alert("hai"+V);
if (isNaN(V)) {
$(this).val(V.replace(/[^0-9]/g,''));
}
}, false);
*
It is working if the alert is there, else it is not working. So any suggestion to solve this.

function numericInput(inputId){
$('#' + inputId).keyup(function (e) {
var regex = new RegExp("^[0-9]+$");
if (!regex.test($(this).val())) {
$(this).val($(this).val().slice(0, -1));
e.preventDefault();
return false;
}
});
}

HTML5 restricting input characters

Is it possible to restrict the input of certain characters in HTML5/JavaScript? For example, could I have an input textbox on the screen and if the user tries to type a letter in it, it wouldn't show up in the box because I've restricted it to only numbers?
I know you can use a pattern which will be checked on submit, but I want the "bad" characters to just never be entered at all.

The input textbox
<input type="text" onKeyDown="myFunction()" value="" />
JavaScript
function myFunction() {
var e = event || window.event; // get event object
var key = e.keyCode || e.which; // get key cross-browser
if (key < 48 || key > 57) { //if it is not a number ascii code
//Prevent default action, which is inserting character
if (e.preventDefault) e.preventDefault(); //normal browsers
e.returnValue = false; //IE
}
}

Use html5 pattern attribute for inputs:
<input type="text" pattern="\d*" title="Only digits" />
OR
Use html5 number type for input :
<input type="number" />

To slightly improve off of jonhopkins excellent answer, I added backspace and delete key acceptance like so:
function inputValidate(){
var e = event || window.event;
var key = e.keyCode || e.which;
if (((key>=48)&&(key<=57))||(key==8)||(key == 46)) { //allow backspace //and delete
if (e.preventDefault) e.preventDefault();
e.returnValue = false;
}
}

For Restricting Characters symbols like '-' and ','
<input type="text" pattern="[^-,]+">
for restricting numbers
<input type="text" pattern="[^0-9]+">
for restricting letters of the alphabet
<input type="text" pattern="[^a-zA-Z]+">

KeyboardEvent.keyCode is deprecated, so here's a solution using the HMLElement.input event. This solution uses a simple regex, and handles copy-paste nicely as well by just removing the offending elements from any input.
My regex: /[^\w\d]/gi
Matches anything not (^) a word character (\w: a-z) or a digit (\d: 0-9).
g modifier makes regex global (don't return after first match)
i modifier makes regex case insensitive
With this regex, special characters and spaces won't be allowed. If you wanted to add more, you'd just have to add allowed characters to the regex list.
function filterField(e) {
let t = e.target;
let badValues = /[^\w\d]/gi;
t.value = t.value.replace(badValues, '');
}
let inputElement = document.getElementById('myInput');
inputElement.addEventListener('input', filterField);
<input id="myInput" type="text" style="width: 90%; padding: .5rem;" placeholder="Type or paste (almost) anything...">

//improved wbt11a function
function numberFieldStrictInput(allowcomma, allownegative) {
var e = event || window.event; // get event object
var key = e.keyCode ||`enter code here` e.which; // get key cross-browser
if(key==8 || key==46 || key == 9 || key==17 || key==91 || key==18 ||
key==116 || key==89 || key==67 || key==88 || key==35 || key==36) //back, delete tab, ctrl, win, alt, f5, paste, copy, cut, home, end
return true;
if(key == 109 && allownegative)
return true;
if(key == 190 && allowcomma)
return true;
if(key>=37 && key<=40) //arrows
return true;
if(key>=48 && key<=57) // top key
return true;
if(key>=96 && key<=105) //num key
return true;
console.log('Not allowed key pressed '+key);
if (e.preventDefault) e.preventDefault(); //normal browsers
e.returnValue = false; //IE
}
//on input put onKeyDown="numberFieldStrictInput(1,0)"

What about this (it supports special keys, like copy, paste, F5 automatically)?
function filterNumericInput() {
var e = event || window.event; // get event object
if (e.defaultPrevented) {
return;
}
const key = e.key || e.code;
if ((e.key.length <= 1) && (!(e.metaKey || e.ctrlKey || e.altKey))) {
if (!((key >= '0' && key <= '9') || (key === '.') || (key === ',') || (key === '-') || (key === ' '))) {
if (e.preventDefault) {
e.preventDefault();
} else {
e.returnValue = false;
}
}
}
}

Limit input to letters, numbers and '.' (for React users only)
Here is my simple solution, I couldn't find a better solution for React and made my own. 3 steps.
First, create a state.
const [tagInputVal, setTagInputVal] = useState("");
Then, use the state as input value (value={tagInputVal}) and pass the event to the onChange handler.
<input id="tag-input" type="text" placeholder="Add a tag" value={tagInputVal} onChange={(e) => onChangeTagInput(e)}></input>
Then, set the value of the event inside onChange handler.
function onChangeTagInput(e) {
setTagInputVal(e.target.value.replace(/[^a-zA-Z\d.]/ig, ""));
}

var keybNumberAndAlpha = new keybEdit(' 0123456789abcdefghijklmnopqrstuvwxyz');
function keybEdit(strValid, strMsg) {
var reWork = new RegExp('[a-z]','gi'); // Regular expression\
// Properties
if(reWork.test(strValid))
this.valid = strValid.toLowerCase() + strValid.toUpperCase();
else
this.valid = strValid;
if((strMsg == null) || (typeof(strMsg) == 'undefined'))
this.message = '';
else
this.message = strMsg;
// Methods
this.getValid = keybEditGetValid;
this.getMessage = keybEditGetMessage;
function keybEditGetValid() {
return this.valid.toString();
}
function keybEditGetMessage() {
return this.message;
}
}
function editKeyBoard(ev, objForm, objKeyb) {
strWork = objKeyb.getValid();
strMsg = ''; // Error message
blnValidChar = false; // Valid character flag
var BACKSPACE = 8;
var DELETE = 46;
var TAB = 9;
var LEFT = 37 ;
var UP = 38 ;
var RIGHT = 39 ;
var DOWN = 40 ;
var END = 35 ;
var HOME = 35 ;
// Checking backspace and delete
if(ev.keyCode == BACKSPACE || ev.keyCode == DELETE || ev.keyCode == TAB
|| ev.keyCode == LEFT || ev.keyCode == UP || ev.keyCode == RIGHT || ev.keyCode == DOWN) {
blnValidChar = true;
}
if(!blnValidChar) // Part 1: Validate input
for(i=0;i < strWork.length;i++)
if(ev.which == strWork.charCodeAt(i) ) {
blnValidChar = true;
break;
}
// Part 2: Build error message
if(!blnValidChar)
{
//if(objKeyb.getMessage().toString().length != 0)
// alert('Error: ' + objKeyb.getMessage());
ev.returnValue = false; // Clear invalid character
ev.preventDefault();
objForm.focus(); // Set focus
}
}
<input type="text"name="worklistFrmDateFltr" onkeypress="editKeyBoard(event, this, keybNumberAndAlpha)" value="">

I found that onKeyDown captures Shift key, arrows, etc. To avoid having to account for this, I could filter out character input easily by subscribing to onKeyPress instead.

Since many of the answers above didn't satisfy me, I propose my solution which solves the problem of the input event being uncancelable by storing the previous value in a custom attribute, and restoring it in case the pattern is not matched:
const input = document.querySelector('#input-with-pattern')
input.addEventListener('keyup', event => {
const value = event.target.value;
if (!/^[a-zA-Z]+$/.test(value) && value !== '') { // it will allow only alphabetic
event.target.value = event.target.getAttribute('data-value');
} else {
event.target.setAttribute('data-value', value);
}
});
<input id="input-with-pattern">

event key code for ampersand?

I am trying to find the keycode for ampersand and underscore. I should not allow my users to allow to enter ampersands and underscores. I was taking a look at one list, and it mentions 55 as the keycode for both 7 & ampersand, and another list says that 55 is the keycode for 7. So if I return false when my user hits the keycode 55, I am disallowing the user from using 7, which isn't the requirement. How do I find the keycodes for ampersand and underscore?
I just tried with 55, but it only is giving me the alert for 7 not with ampersand!
function noenter(e)
{
evt = e || window.event;
var keyPressed = evt.which || evt.keyCode;
if(keyPressed==13)
{
return false;
}
else if(evt.shiftKey && keyPressed===55)
// else if(keyPressed==59 || keyPressed==38 || keyPressed==58 || keyPressed==95)
{
alert("no special characters");
return false;
}
}

Use the keypress event and test directly for the character as follows. Don't mess with key codes: they will vary between different keyboard types and cultures. Character codes won't.
var el = document.getElementById("your_input");
el.onkeypress = function(evt) {
evt = evt || window.event;
var charCode = evt.which || evt.keyCode;
var charStr = String.fromCharCode(charCode);
if (charStr == "&" || charStr == "_") {
alert(charStr);
return false;
}
};

Check if the shift key is down:
//e = Event
(e.shiftKey && e.keyCode === 55) //returns true if Shift-7 is pressed

ok got it! it is 38! sorry for putting up this question!

I've solved it using the Unicode Key Identifier. Below is my implementation with jQuery:
function parseKey(key) {
return parseInt(key.substring(2), 10);
}
$inputs.bind('keydown', function(e) {
var c=parseKey(e.originalEvent.keyIdentifier);
//allow only numbers and backspace
if ((c<30 || c>39) && e.which!=8)
e.preventDefault();
if (e.which == 13)
$(this).blur();
});