Examples

The converter

Own this code rather than depending on an editor mode: forum software disagrees about BBCode dialects, so the mapping belongs where you can adjust it.

function htmlToBBCode(html) {
    const root = new DOMParser().parseFromString(html, "text/html").body;

    const TAGS = { b: "b", strong: "b", i: "i", em: "i", u: "u",
                   s: "s", strike: "s", del: "s" };

    function walk(node) {
        if (node.nodeType === 3) return node.nodeValue;                 // text
        if (node.nodeType !== 1) return "";

        const tag = node.tagName.toLowerCase();
        const inner = Array.from(node.childNodes).map(walk).join("");

        if (TAGS[tag]) return `[${TAGS[tag]}]${inner}[/${TAGS[tag]}]`;
        if (tag === "a") return `[url=${node.getAttribute("href") || ""}]${inner}[/url]`;
        if (tag === "img") return `[img]${node.getAttribute("src") || ""}[/img]`;
        if (tag === "blockquote") return `[quote]${inner.trim()}[/quote]\n`;
        if (tag === "pre" || tag === "code") return `[code]${inner}[/code]`;
        if (tag === "ul") return `[list]\n${inner}[/list]\n`;
        if (tag === "ol") return `[list=1]\n${inner}[/list]\n`;
        if (tag === "li") return `[*]${inner.trim()}\n`;
        if (tag === "br") return "\n";
        if (tag === "p" || tag === "div") return inner + "\n\n";

        // Inline colour and size survive as style attributes.
        const colour = node.style && node.style.color;
        if (colour) return `[color=${colour}]${inner}[/color]`;

        return inner;
    }

    return walk(root).replace(/\n{3,}/g, "\n\n").trim();
}

// Call it with whatever the editor saved:
const bbcode = htmlToBBCode(editor.getHTMLCode());

Anything the editor produces that BBCode cannot express (tables, nested layouts, embedded media) has no faithful equivalent — decide explicitly whether to drop it or render it as a link.

Getting HTML content from the editor