Showdown: parse single underscores as italics

public/script.js

@@ -172,6 +172,7 @@ import {
 } from './scripts/secrets.js';
 import { EventEmitter } from './lib/eventemitter.js';
 import { markdownExclusionExt } from './scripts/showdown-exclusion.js';
+import { markdownUnderscoreExt } from './scripts/showdown-underscore.js';
 import { NOTE_MODULE_NAME, initAuthorsNote, metadata_keys, setFloatingPrompt, shouldWIAddPrompt } from './scripts/authors-note.js';
 import { registerPromptManagerMigration } from './scripts/PromptManager.js';
 import { getRegexedString, regex_placement } from './scripts/extensions/regex/engine.js';
@@ -690,6 +691,7 @@ function reloadMarkdownProcessor(render_formulas = false) {
             parseImgDimensions: true,
             tables: true,
             underline: true,
+            extensions: [markdownUnderscoreExt()],
         });
     }
 

public/scripts/showdown-underscore.js

@@ -0,0 +1,22 @@
+// Showdown extension that replaces words surrounded by singular underscores with <em> tags
+export const markdownUnderscoreExt = () => {
+    if (!canUseNegativeLookbehind()) {
+        console.log('Showdown-underscore extension: Negative lookbehind not supported. Skipping.');
+        return [];
+    }
+
+    return [{
+        type: 'lang',
+        regex: /\b(?<!_)_(?!_)(.*?)(?<!_)_(?!_)\b/g,
+        replace: '<em>$1</em>',
+    }];
+};
+
+function canUseNegativeLookbehind() {
+    try {
+        new RegExp('(?<!_)');
+        return true;
+    } catch (e) {
+        return false;
+    }
+}