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How to detect actual characters only with JS key-events (for text-editor)

I am working on a WYSIWYG-like editor for equations (in combination with plain text).
Since implementing the functionalities (for equations) that I'd like to have won't work using the existing frameworks (contentEditable or document.designMode), I am now building an editor from scratch. (so far it has worked out good, I've successfully implemented most functionalities of a normal editor)
I've been using the keydown event to detect user input while in "edit-mode" (that is, when the user has clicked on the editor-area, also displaying the cursor), but the problem with that is, that when clicking "alt" or "strg" or other keys that aren't actual characters, they'll also get displayed in my editor.
Now, what I've tried is to ignore those keys by using if-statements, but there are 2 issues I see with that:
1. It may influence performance, when too many keys have to be ignored
2. I can never be sure, if there doesn't exist some exotic key, perhaps on a Mac or so, which I didn't ignore
I have also tried to use the keypress event instead, which worked mostly fine, but still displayed "Enter" and "Delete", so who knows what others keys it may display too. Also, it is labelled as "deprecated".
Are there any better ways of doing that, or will I just have to make a big list of keys to ignore?

A simple (but limited) approach would be to check if the keydown event's keyCode is between 65 (code for key 'a') and 90 (code for 'z').
If you need more than just letters though, another solution would be to check the event's key and its length. For actual characters, the key simply holds that character (so length 1). For Ctrl, Shift, etc., key will hold the full name of the key, e.g. "Control", "Shift", etc.
So, if a key is an actual character, the key property will have a length of 1.
document.onkeydown = function(e) {
var keycode = e.keyCode;
if(e.key.length == 1) {
document.querySelector("#editor").innerHTML += e.key;
}
}
<p id="editor"></p>

You can check this one too
document.addEventListener("keydown", (event) => {
console.log(event);
if (event.code === "Escape") {
console.log("abc");
}
});

Get key pressed from DOM input event (or equivalent solution)

Background:
I am writing a script that does some stuff when a user clears an input type="search" (essentially the same as type=text) using backspace or delete. I want to address the case where user highlights original text and starts typing new text, and also pasting.
Because of this I cannot use keydown (fires before input value is changed). Cannot use keypress (need to fire when backspace/delete is pressed, and it fires too early as well). Using keyup is bad because I can't clear when value === 1 (might already be several characters in the field). I can build in some slop but then it won't clear right away which looks buggy.
Question:
So the DOM input event fires right when the value is updated, which is exactly what I want (tested using jQuery on 'input'). However, I cannot find the captured key (which I need to differentiate between deleting and entering content). I couldn't find great info on the spec. It seems like this is going to be implemented in the data section of the event, which does not yet have any browser support. Does that mean it is currently impossible to get the key from the oninput event? If so, how do most developers handle this? I imagine wanting the key immediately after it is entered is very common, surely there is a decent solution for this?
Current implementation for the curious:
For the time being I am grabbing the value right after keydown by using a setTimeout of 0. This gets me both the key and the updated value, but feels dirty and requires extra handlers for onpaste and such. While writing this it occurs to me I could probably use oninput by keeping track of the last value and comparing to current to differentiate between entering and deleting, but that doesn't seem all that much better, and would still require a separate paste handler since I want my event to fire every time a user pastes, and it doesn't seem to have any flags that would let me know the event was a paste.

I decided the last suggestion of dandavis was the best workaround. I used keydown to grab key info and triggered the action on paste, empty field, or 1 character and last character was not backspace/delete (on input does not fire on enter, modifier keys, etc. so those don't need to be accounted for).
The code looks like this:
var onClear = function(action){
var lastKey;
jQueryObject.keydown(function(e){
lastKey = e.which;
});
jQueryObject.on('input', function(){
if( !this.value.length || (this.value.length === 1 && lastKey !== 46 && lastKey !== 8) ){
action();
}
});
jQueryObject.on('paste', function(){
action();
});
}

JavaScript keypress event get end value of textarea

I was wondering if it were possible to get the end result in the keypress event?
Currently, I am using the keyup because it is activated after the user has done text editing in a texteara, but I have written a method that does something similar using the Mootools library:
input.addEvent("keypress", function (input) {
var previous_result = this.value;
var end_result = this.value + input.key;
});
However, this method is horrible when dealing with special keys such as backspace, or if the user chooses to use CTRL + a && Backspace in which case the value of the input element would not be "an empty string".
I'm curious because I have observed Google's search engine sending XMLHttpRequest AND mutating the page before the keyup event triggered.
Additionally, the input they use manages to overcome my problem of removing entire text while still enjoying the luxury of keypress.

This will make it work:
input.addEvent( "keypress", function ( input ) {
setTimeout(function () {
input.value // returns the updated value
}, 0 );
});
Live demo: http://jsfiddle.net/yQQ5P/ (I use built-in API, as I don't know Mootools)
So, you use a zero-timeout which acts as a yield. This gives the browser's UI thread an opportunity to updated the input's value.

Preventing values in html inputs with jQuery

How do I prevent the user from changing a string in an input box if the combined value of that input with their latest keypress does not pass a regular expression?
I see all kinds of examples on how to use keypress for testing individual characters, but I need to match the entire value to a regular expression, not just the key they pressed.
For example, The textbox needs to conform to the following regular expression:
"^023-[0-9]{0,7}$"
So if they want to change "023" to "23", it should stop them when they delete the 0 character. The same is true for deleting the - after 023. It should also stop them once they enter more than 7 numbers after 023. This cannot be done on blur. It must be done after every key stroke. Performance is not an issue.
If I use keypress() event in jQuery, and get the value of the input element like this:
$(this).val()
Then I will only get the value before they pressed the key - not after. Thus, there is no way to test the input against my regular expression. I cannot simply append the key pressed to this string, because I cannot make the assumption that they are only entering keys at the right-most side of the string.
I have looked into keydown/keyup events, and while keyup seems to give me the current value of the input after the user has pressed a key, I am finding it difficult to remove the effects of what they typed... which keypress() does not have a problem with apparently.
var regex = new RegExp("^023-[0-9]{0,7}$");
$("#number").keyup(function(event) {
var number = $(this).val();
if(!regex.test(number)) {
event.preventDefault();
return false;
}
});
The above code just doesn't work. It seems keypress() gives me the ability to stop what they typed, but keyup gives me the ability to get the current value. I need a solution that does both ;)
The problem really stems from the fact that the browser has no MVC architecture. The model is the view. Unfortunately, we can't validate the updated model before the view is updated... because we need the view updated during a keyup event to get the updated model data... and by then, it's too late to prevent them from updating the view since it's already been updated.

How about this:
var prevValue = "";
$("#number").keydown(function(e) {
prevValue = $(this).val();
});
$("#number").keyup(function(e) {
...
if(!regex.test(number))
$(this).val(prevValue);
// show error or other processing you need to do
});

try this:
var pattern = new RegExp("^023-[0-9]{0,7}$");
$("#number").change(function(e){
if (!pattern.test($(this).val())) {
return false
}
})

The event parameter that you have will contain all the details of the event that you are handling. This will include the key that was pressed.
Of note is that keydown and keyup events will return the key pressed, while keypress will return which character is entered which may be better for your choice.
See the help page here: http://api.jquery.com/keypress/ for more details.
In summary though event.which should do what you want. You can combine that with the original value of the textbox and from that get the new value. If it isn't valid then cancel the event, if it is valid then let it go...

If you can use the jQuery Validate plug-in, you can create a custom validator to match your RegEx. Then all you have to do is map the validator to that input. You'll still have to validate on the server-side, but you're probably already doing that.

To do what you want, the jquery caret plugin is required. Please note that you'll need to remove the comments from the beginning of the file (or at least move them) or the byte order mark at the start of the code will cause an error.
So, what you need to do is capture the keypress, insert the character at the correct place in the string, check if that new string is valid and then either show the new insertion or not. I think the following does what you need:
$(document).ready(function(){
var regex = new RegExp("^023-[0-9]{0,7}$");
var caretpos;
$('#number').keypress(function(event){
// get the caret position
caretpos = $(this).caret().start;
//split the current value according to where the caret is
valuestart = $(this).val().substring(0,caretpos);
valueend = $(this).val().substring(caretpos);
// insert the fired character into the string
valuetocheck = valuestart + String.fromCharCode(event.which) + valueend;
// check if the proposed new value passes the regex
if (!regex.test(valuetocheck)) {
// failed, prevent the character being shown
event.preventDefault();
}
});
});
I've tested this in the latest versions of FF, Chrome, Safari, Opera and IE9. In FF and Opera, cursor key presses are completely ignored while they move the caret as usual in the other browsers so you may want to add some code to ensure consistent behaviour for that. You may also want to add some code to handle what will happen if the user selects some characters rather than just clicking into the input. The jquery caret plugin has usage examples for selections.

JS/jQuery : Best way to get typed characters after '#' in textarea

I am creating a smart textarea that needs to recognise a term typed after the '#' symbol.This needs to be able to work for the term CURRENTLY being typed and be able to work for multiple instances of the '#' symbol in a single textarea.
It is designed to work the same way as Facebook when you type the '#' symbol to tag a person in a post. A drop down list will appear and list items will get filtered depending on the term after the '#' symbol.
This also needs to work if the user were to type a term then amend it later. I understand this complicates things a little.
What is the best way to achieve this functionality?

I don't know if it helps but here's i small script to find the hashes.
http://jsfiddle.net/aNgVV/

I suggest you look at the jQuery UI demo for the Autocomplete widget, specifically the demo for using a remote datasource with cache. Specifically for the following reasons:
It automatically takes care of the drop-down widget you mentioned.
It demonstrates how you can populate that drop-down with items based on an AJAX call (which I presume you need).
The demo for Autocomplete caching parses the text in the INPUT element, as it tries to determine whether or not the value the user is currently typing has already been cached, and reacts accordingly. I assume you can do something similar to check for the # types, and to check if a previous # tag is being modified as well.

Initially you need to catch the '#' key being pressed and then capture the subsequent key presses and pass them to a function to handle your auto completion requirements. A rough outline to of the code is below. You may need to catch whitespace key presses as well to stop the auto-completion.
var hashKeyPressed = false;
$('#TextArea').keyup(function(event) {
if(event.keyCode == '222') {
// this will catch the '#' key
hashKeyPressed = true;
}
if(hashKeyPressed) {
// Here you can start build up subsequent key presses into a string
// and pass them to a function to handle the auto-completion
}
});

You can capture the keyup event and check what has been entered last like so:
$('#myTextArea').keyup(function () {
var len = $(this).val().length;
if ($(this).val().substring(length - 1, 1) == '#') {
// Do whatever you want to do here
}
});
EDIT:
You are right - you could do it this way instead:
$('#myTextArea').keypress(function (e) {
if (e.which == 222) {
// do something here
}
});