How to allow only numeric (0-9) in HTML inputbox using jQuery?

Refactored the accepted answer so that comments no longer need to used. Also this is easier to test with jasmine.

    allowBackspaceDeleteTabEscapeEnterPress: function(event){
    return ($.inArray(event.keyCode, [46, 8, 9, 27, 13, 190]) >= 0);
},
allowContorlAPress: function(event){
    return (event.keyCode == 65 && event.ctrlKey === true)
},
allowHomeEndLeftRightPress: function(event){
    return (event.keyCode >= 35 && event.keyCode <= 39)
},
theKeyPressedIsEditRelated: function (event) {
    return (this.allowBackspaceDeleteTabEscapeEnterPress(event)
            || this.allowContorlAPress(event)
            || this.allowHomeEndLeftRightPress(event));
},
isNotFromTheNumKeyPad: function (event) {
    return (event.keyCode < 96 || event.keyCode > 105);
},
isNotFromTopRowNumberKeys: function (event) {
    return (event.keyCode < 48 || event.keyCode > 57);
},
theKeyIsNonNumeric: function (event) {
   return (event.shiftKey
           || (this.isNotFromTopRowNumberKeys(event)
                && this.isNotFromTheNumKeyPad(event)));
},
bindInputValidator: function(){
    $('.myinputclassselector').keydown(function (event) {
        if(this.validateKeyPressEvent(event)) return false;
    });
},
validateKeyPressEvent: function(event){
    if(this.theKeyPressedIsEditRelated(event)){
        return;
    } else {
        if (this.theKeyIsNonNumeric(event)) {
            event.preventDefault();
        }
    }
}

This is for alpha-numeric validation and will take care of pasting also:

//Input Validation
var existingLogDescription = "";

$('.logDescription').keydown(function (event) {
    existingLogDescription = this.value;

});


$('.logDescription').keyup(function () {
    if (this.value.match(/[^a-zA-Z0-9 ]/g)) {
        alert("Log Description should contain alpha-numeric values only");
        this.value = this.value.replace(/[^a-zA-Z0-9 ]/g, '');
        this.value = existingLogDescription;
    }
});

I recommend to check event.metaKey as well. If that's set to true, the user might be doing something like Cmd+A to select all the text in the field. You should allow that too.

There are so many good answers to doing it with JavaScript or jQuery here.

I will add a very easy way to archive this using just HTML5:

<input type="number" name="quantity" min="0" max="9">

Put a class in input text and name it only_numbers.

Put the jQuery code in the page:

$(document).ready(function() {
    $('.only_numbers').keyup(function() {
        var numbers = $(this).val();
        $(this).val(numbers.replace(/\D/, ''));
    });
});

Allow single decimal and numeric only, single line:

$(document).on("input", "#bill_amount", function() {
    this.value = this.value.replace(/[^0-9.]/g, '').replace(/(\..*)\./g, '$1');
    // Remove extra dots by (/(\..*)\./g, '$1');
});

Short answer:

$('#mobile_number').on('input', function() {
            // Remove non-numeric characters
            this.value = this.value.replace(/\D/g, '');
            if (this.value.length > 8) {
                this.value = this.value.slice(0, 8);
            }
});

This code ensures that the user can only type numeric characters (0–9) into input fields with the class .classname. All other keys (letters, symbols) are blocked:

$(document).on("keypress", ".classname", function(evt) {
    evt = (evt) ? evt : window.event;
    var charCode = (evt.which) ? evt.which : evt.keyCode;
    if (charCode > 31 && (charCode < 48 || charCode > 57)) {
        return false;
    }
    return true;
});

Use the jQuery numeric value. Below function allows for decimal and numeric values.
Example:

$("#inputId").numeric({ allow: "." });

This will work on all browsers:

function Numbers(e)
{
    if($.browser.msie)
    {
        if(e.keyCode > 47 && e.keyCode < 58)
            return true;
        else
            return false;
    }
    else
    {
        if((e.charCode > 47 && e.charCode < 58) || (e.charCode == 0))
            return true;
        else
            return false;
    }
}