If you want to restrict input (as opposed to validation), you could work with the key events. something like this:

<input type="text" class="numbersOnly" value="" />

And:

jQuery('.numbersOnly').keyup(function () {     this.value = this.value.replace(/[^0-9\.]/g,'');});

This immediately lets the user know that they can't enter alpha characters, etc. rather than later during the validation phase.

You'll still want to validate because the input might be filled in by cutting and pasting with the mouse or possibly by a form autocompleter that may not trigger the key events.

Update

There is a new and very simple solution for this:

It allows you to use any kind of input filter on a text <input>, including various numeric filters. This will correctly handle Copy+Paste, Drag+Drop, keyboard shortcuts, context menu operations, non-typeable keys, and all keyboard layouts.

See this answer or try it yourself on JSFiddle.

jquery.numeric plugin

I've successfully implemented many forms with the jquery.numeric plugin.

$(document).ready(function(){    $(".numeric").numeric();});

Moreover this works with textareas also!

However, note that Ctrl+A, Copy+Paste (via context menu) and Drag+Drop will not work as expected.

HTML 5

With wider support for the HTML 5 standard, we can use pattern attribute and number type for input elements to restrict number only input. In some browsers (notably Google Chrome), it works to restrict pasting non-numeric content as well. More information about number and other newer input types is available here.

I thought that the best answer was the one above to just do this.

jQuery('.numbersOnly').keyup(function () {      this.value = this.value.replace(/[^0-9\.]/g,''); });

but I agree that it is a bit of a pain that the arrow keys and delete button snap cursor to the end of the string ( and because of that it was kicked back to me in testing)

I added in a simple change

$('.numbersOnly').keyup(function () {    if (this.value != this.value.replace(/[^0-9\.]/g, '')) {       this.value = this.value.replace(/[^0-9\.]/g, '');    }});

this way if there is any button hit that is not going to cause the text to be changed just ignore it. With this you can hit arrows and delete without jumping to the end but it clears out any non numeric text.