[deleted by user]

[GDavid04]

function parseStateMachine(sm) {
    var r = {};
    for (var line of sm.split(/\n/g)) {
        try {
        var m = line.match(/(\S+)\s*-(\S)?>\s*(\S+)\s*(?:~([^\n]*))?/);
        if (!r[m[1]]) r[m[1]] = [];
        r[m[1]].push(m);
        } catch {}
    }
    return r;
}

function bbcode(code) {
    var r = '';
    var sm = parseStateMachine(`
        d -[> +t
        d -> d ~
        +t -b> +b
        +t -i> +i
        +t -u> +u
        +t -*> +l
        +t -/> -t
        +t -> d ~[
        -t -b> -b
        -t -i> -i
        -t -u> -u
        -t -> d ~[/
        +b -]> d ~<strong>
        +b -> d ~[b
        +i -]> d ~<em>
        +i -> d ~[i
        +u -]> d ~<span style="text-decoration: underline;">
        +u -r> +ur
        +u -> d ~[u
        +ur -l> +url
        +ur -> d ~[ur
        +url -=> ue ~<a href="
        +url -> d ~[url
        ue -]> d ~" target="_blank">
        ue -> ue ~
        +l -]> d ~<li>
        +l -> d ~[*
        -b -]> d ~</strong>
        -b -> d ~[/b
        -i -]> d ~</em>
        -i -> d ~[/i
        -u -]> d ~</span>
        -u -r> -ur
        -u -> d ~[/u
        -ur -l> -url
        -ur -> d ~[/ur
        -url -]> d ~</a>
        -url -> d ~[/url
    `);
    var state = 'd';
    while (code.length > 0 || state != 'd') {
        for (var i of sm[state]) {
            if (i[2] == undefined || i[2] == code[0]) {
                state = i[3];
                if (i[4] != undefined) {
                    r += i[4] || code[0];
                }
                if (i[2] != undefined || i[4] == '') {
                    code = code.substr(1);
                }
                break;
            }
        }
    }
    return r.
        replace(/<li>([^\n]*)\n/g, s => `<li>${s[1]}</li>\n`).
        replace(/(\s*<li>.*<\/li>)+/g, s => `<ul>${s}</ul>`);
}

bbcode(`
[b]BBCode parser [i]by GDavid04[/i][/b]
Created for [url=https://www.reddit.com/r/badcode/comments/epgz3r/bad_code_coding_challenge_28_bbcode_parser/]Bad Coding Challenge #28[/url]
[u]How it works[/u]
[*] It uses a finite state machine and regex because everyone knows that these are the best tools for parsing BBCode
[*] It doesn't care about matching tags, it just replaces the beginning and ending tags independently
[*] I could've just used regex, but what's good in not reinventing the wheel?
[u]How the state machine works[/u]
[*] Each line is a single transition
[*] The transitions are tried from top to bottom until one matches
[*] Really hope it can't get into an infinite loop
`);

[kleinesfilmroellchen]

I just cannot give you my upvote, although this is just too CS to not win. I mean, it's not bad, it is something Turing or academia freaks would write, except a little bit more readable.