Adding ordered list styles to markdown parser

I am using Marked.js for parsing Markdown. The original Markdown language does not support different ordered list styles, for instance, lettered ones, like HTML. I would like to introduce this function to the parser. Below is my JavaScript code:

const customList = {
    name: "customList",
    level: "block",
    start(src) {
        return src.match(/(?:^|\n)(\s*)(-|\+|\*|\d+\.|[A-Za-z]+\.|[iIvVxX]+\.)(\s+)/)?.index;
    },
    tokenizer(src, tokens) {
        const rule = /^([^\S\r\n]*)(-|\+|\*|\d+\.|[A-Za-z]+\.|[iIvVxX]+\.)(\s+)(.*)/;
        const textRule = /^([^\S\r\n]*)(.*)/;
        const match = rule.exec(src);
        let style;
        if (match) {
            if (/-/.test(match[2])) style = "-";
            else if (/\+/.test(match[2])) style = "+";
            else if (/\*/.test(match[2])) style = "*";
            else if (/\d+\./.test(match[2])) style = "1";
            else if (/[IVX]+\./.test(match[2])) style = "I";
            else if (/[ivx]+\./.test(match[2])) style = "i";
            else if (/[A-Z]+\./.test(match[2])) style = "A";
            else if (/[a-z]+\./.test(match[2])) style = "a";
            const items = [];
            let remainingSrc = src;
            let prevIndent = match[1].length;
            let itemText = "", raw = "";
            while (remainingSrc) {
                const itemMatch = rule.exec(remainingSrc);
                const textMatch = textRule.exec(remainingSrc);
                if (itemMatch && itemMatch[1].length === prevIndent) {
                    let itemStyle;
                    if (/-/.test(itemMatch[2])) itemStyle = "-";
                    else if (/\+/.test(itemMatch[2])) itemStyle = "+";
                    else if (/\*/.test(itemMatch[2])) itemStyle = "*";
                    else if (/\d+\./.test(itemMatch[2])) itemStyle = "1";
                    else if (/[IVX]+\./.test(itemMatch[2])) itemStyle = "I";
                    else if (/[ivx]+\./.test(itemMatch[2])) itemStyle = "i";
                    else if (/[A-Z]+\./.test(itemMatch[2])) itemStyle = "A";
                    else if (/[a-z]+\./.test(itemMatch[2])) itemStyle = "a";
                    else break;
                    if (itemStyle !== style) break;
                    if (itemText) {
                        const itemTokens = [];
                        this.lexer.inlineTokens(itemText, itemTokens);
                        items.push({
                            type: "customListItem",
                            raw: raw,
                            tokens: itemTokens
                        });
                    }
                    itemText = itemMatch[4].trim();
                    raw = itemMatch[0];
                    remainingSrc = remainingSrc.slice(itemMatch[0].length + 1);
                }
                else if (textMatch && textMatch[1].length === prevIndent) {
                    itemText += "\n" + textMatch[2];
                    raw += "\n" + textMatch[0];
                    remainingSrc = remainingSrc.slice(textMatch[0].length + 1);
                }
                else break;
            }
            if (itemText) {
                const itemTokens = [];
                this.lexer.inlineTokens(itemText, itemTokens);
                items.push({
                    type: "customListItem",
                    raw: raw,
                    tokens: itemTokens
                });
            }
            const token = {
                type: "customList",
                raw: src.slice(0, src.length - remainingSrc.length),
                style: style,
                items: items
            };
            return token;
        }
    },
    renderer(token) {
        const listItems = token.items.map(item =>
            `<li>${this.parser.parseInline(item.tokens)}</li>`
        ).join('\n');
        if (token.style === "-" || token.style === "+" || token.style === "*") return `<ul>\n${listItems}\n</ul>\n`;
        else if (token.style === "1" || token.style === "I" || token.style === "i" || token.style === "A" || token.style === "a") return `<ol type="${token.style}">\n${listItems}\n</ol>\n`;
    },
    childTokens: ["items"]
};
marked.use({ extensions: [customList] });

It works normally in most cases:

Example 1

marked.parse(" 1. first\n 2. second\n 3. third")

Output

<ol type="1">
    <li>first</li>
    <li>second</li>
    <li>third</li>
</ol>

Example 2

marked.parse("A. first\nB. second\n* first\n* second")

Output

<ol type="A">
    <li>first</li>
    <li>second</li>
</ol>
<ul>
    <li>first</li>
    <li>second</li>
</ul>

However, when I try this:

marked.parse("1 + 1")

The output becomes:

<p>1</p>
<ul>
    <li>1</li>
</ul>

I expect that only the list markers appearing at the beginning of a line (surrounded by whitespaces) will be detected as a list.

I tried getting some values of the variables by adding console.log(src) in the tokenizer function. Surprisingly, I receive two outputs when I try the above case.

1 + 1
 + 1

Why is this occurring? I am sorry that I am new to JavaScript, and the code may look clumsy. If you need more information, please let me know. Thanks a lot.