SillyTavern/public/scripts/showdown-underscore.js

// Showdown extension that replaces words surrounded by singular underscores with <em> tags
export const markdownUnderscoreExt = () => {
    try {
        if (!canUseNegativeLookbehind()) {
            console.log('Showdown-underscore extension: Negative lookbehind not supported. Skipping.');
            return [];
        }

        return [{
            type: 'lang',
            regex: new RegExp('\\b(?<!_)_(?!_)(.*?)(?<!_)_(?!_)\\b', 'g'),
            replace: '<em>$1</em>',
        }];
    } catch (e) {
        console.error('Error in Showdown-underscore extension:', e);
        return [];
    }
};

function canUseNegativeLookbehind() {
    try {
        new RegExp('(?<!_)');
        return true;
    } catch (e) {
        return false;
    }
}