Userscript for easily replying in Stack Exchange chat

In SE chat, there are two different ways to mention people: A "ping" to just get a user's attention, and a "reply" that points out a specific message by the user. This script makes it so that beginning a message with (by default) @username@, the message is made a reply to username's most recent message. Additionally, any message that begins with @username* is never sent, but username's last message is starred.

I have very little experience with Javascript, and even less with jQuery. I wrote this code flying by the seat of my pants. I googled a lot of things, referenced a lot of things used in this script, and guessed at a lot of others. It works, sure, but that last sentence doesn't inspire a lot of confidence. I don't really have anything specifically I want to ask about, since I don't know what stuff it is that I don't know! :P That being said, I'm sure there are probably a lot of problems with it that someone more experienced than I would be able to identify.

// ==UserScript==
// @name easyreply
// @namespace http://reddit.com/u/undergroundmonorail
// @author monorail
// @version 0.2
// @description easily respond to a user's last message in se chat
// @grant none
// @copyright MIT
// @include *://chat.stackoverflow.com/*
// @include *://chat.stackexchange.com/*
// @include *://chat.meta.stackexchange.com/*
// ==/UserScript==

// workaround for Safari
// (as if anyone uses Safari)
// ((glares at @AlexA.))
function withJQuery(f) {
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.textContent = '('+ f + ')(jQuery)';
    document.head.appendChild(script);
}

withJQuery(easyreply);

function easyreply($) {
    // OPTIONS
    // prefix/suffix of username to reply to
    var R_PREFIX = '@';
    var R_SUFFIX = '@';
    // prefix/suffix of username to star
    var S_PREFIX = '@';
    var S_SUFFIX = '*';

    var input = $('#input');

    function messageAuthor(message) {
        // a group of continuous messages by the same user is a "monologue"
        // go up to the monologue, then find the author
        // strip spaces from the username because that's how the name you type to ping someone is found
        // (e.g.) pinging 'Alex A.' is done by typing '@AlexA.')
        return message.parent().parent().children('.signature').children('.username').text().replace(/ /g, '');
    }

    function lastMessageFrom(author) {
            // go backwards through the messages until finding one that is by the mentioned user OR there are no more messages
            var message = $('.message:last');
            while (message.length && author.toLowerCase() !== messageAuthor(message).toLowerCase()) {
                var prev = message.prev('.message'); // go up one message
                if (!prev.length) { // handle monologue barriers
                    // stole this code wholesale from doorknob. i mostly understand it.
                    prev = message.closest('.monologue').prevAll('.monologue').eq(0).find('.message:last');
                }
                message = prev;
            }
            return message; 
    }

    function replyToMessage(message) {
        if (message.length) { // if a message was found
            // reply syntax is ':[message id]'
            // messages have the attribute 'id', containing 'message-[message id]'
            // put a colon, everything after the hyphen in the id, then the original message minus the first word
            input[0].value = ':' + message.attr('id').split('-')[1] + ' ' + input[0].value.split(' ').slice(1).join(' ');
        }
    }
1   
    function starMessage(message) {
        message.find('.stars:first .vote').click();
    }

    input.keydown(function(e) {
        if (e.which === 13) { // when enter key is pressed
            e.preventDefault();

            var firstWord = input[0].value.split(' ')[0];
            var R_author = firstWord.slice(R_PREFIX.length, -R_SUFFIX.length); // author name if reply
            var S_author = firstWord.slice(S_PREFIX.length, -R_SUFFIX.length); // author name if star

            if (R_PREFIX + R_author + R_SUFFIX === firstWord) {
                replyToMessage(lastMessageFrom(R_author));
            } else if (S_PREFIX + S_author + S_SUFFIX === firstWord) {
                starMessage(lastMessageFrom(S_author));
                input[0].value = ''; // no message is sent, clear input box manually
            }
        }
    });

    // i don't know what this means but doorknob's code has it ¯\_(ツ)_/¯
    var kdevts = $('#input').data('events').keydown;
    kdevts.unshift(kdevts.pop());
    // doorknob: "For the record, it moves the last keydown event to the front of the 'list' of events."
    // "That way your function is called before SE's default functions."
    // "Otherwise, the message would be sent *before* the replacement would be made."
    // thanks doorknob
}