feat: extend regex to cover more cases (#1338)

maximilianfalco · web-flow · commit 3e8efc8e1882 · 2026-02-18T23:28:14.000+11:00
[![PR App][icn]][demo] | Fix RM-XYZ :-------------------:|:----------: ## 🧰 Changes This fixes CX2834 (mentioned here to not trigger Linear bot). Essentially our regex to normalize malformed emphasis syntaxes were not equipped well enough to handle having the `_` and `__` WITHIN the text element itself. ie things like this were failing since it though the middle `_` was the end ``` _hello_world _ ``` Before | After -- | -- <img width="1582" height="1592" alt="Screenshot 2026-02-13 at 15 36 38" src="https://github.com/user-attachments/assets/19b8c9f5-7887-4cd2-ad0e-b657e9461924" /> | <img width="1586" height="1592" alt="Screenshot 2026-02-13 at 15 36 22" src="https://github.com/user-attachments/assets/f95720fd-2711-44f6-b84a-2e2aec9c632b" /> > [!NOTE] > this actually improves on the original legacy capabilities by allowing us to parse the `__` bold syntax as well > [!IMPORTANT] > we do not extend to also be the same for the `*` syntax simply because the legacy engine didnt support that as well. goal of these malformed transformations was for parity with legacy ## 🧬 QA & Testing Stuff like ``` _hello_world _ ``` or even ``` __hello__world __ ``` should be considered one node and not split into two - [Broken on production][prod]. - [Working in this PR app][demo]. [demo]: https://markdown-pr-PR_NUMBER.herokuapp.com [prod]: https://SUBDOMAIN.readme.io [icn]: https://user-images.githubusercontent.com/886627/160426047-1bee9488-305a-4145-bb2b-09d8b757d38a.svg
diff --git a/__tests__/transformers/normalize-malformed-md-syntax.test.ts b/__tests__/transformers/normalize-malformed-md-syntax.test.ts
@@ -258,9 +258,7 @@ describe('normalize-malformed-md-syntax', () => {
 
       const paragraph = tree.children[0] as Paragraph;
       const strongNodes = paragraph.children.filter((c): c is Strong => c.type === 'strong');
-      const worldNode = strongNodes.find(
-        n => n.children[0]?.type === 'text' && n.children[0].value === 'World',
-      );
+      const worldNode = strongNodes.find(n => n.children[0]?.type === 'text' && n.children[0].value === 'World');
       expect(worldNode).toStrictEqual({ type: 'strong', children: [{ type: 'text', value: 'World' }] });
     });
   });
@@ -525,6 +523,33 @@ describe('normalize-malformed-md-syntax', () => {
       expect(emphasis).toStrictEqual({ type: 'emphasis', children: [{ type: 'text', value: 'some_snake_case' }] });
     });
 
+    it('should handle underscore bold with double underscores in content', () => {
+      const md = '__hello__world __';
+      const tree = processor.parse(md);
+      processor.runSync(tree);
+      removePosition(tree, { force: true });
+
+      expect(tree.children[0]).toStrictEqual({
+        type: 'paragraph',
+        children: [{ type: 'strong', children: [{ type: 'text', value: 'hello__world' }] }],
+      });
+    });
+
+    it.each([
+      ['_/clients/clear_whitelist _', '/clients/clear_whitelist'],
+      ['_some_text _', 'some_text'],
+      ['_a_b_c _', 'a_b_c'],
+    ])('should handle underscore italic with underscores in content (%s)', (md, expectedContent) => {
+      const tree = processor.parse(md);
+      processor.runSync(tree);
+      removePosition(tree, { force: true });
+
+      expect(tree.children[0]).toStrictEqual({
+        type: 'paragraph',
+        children: [{ type: 'emphasis', children: [{ type: 'text', value: expectedContent }] }],
+      });
+    });
+
     it('should handle malformed bold with word before and snake_case content', () => {
       const md = 'Find** some_snake_case** and click';
       const tree = processor.parse(md);
diff --git a/processor/transform/mdxish/normalize-malformed-md-syntax.ts b/processor/transform/mdxish/normalize-malformed-md-syntax.ts
@@ -24,13 +24,15 @@ const asteriskBoldRegex =
   /([^*\s]+)?\s*(\*\*)(?:\s+((?:[^*\n]|\*(?!\*))+?)(\s*)\2|((?:[^*\n]|\*(?!\*))+?)(\s+)\2)(\S|$)?/g;
 
 // Pattern for __ bold __
-const underscoreBoldRegex = /([^_\s]+)?\s*(__)(?:\s+((?:[^_\n]|_(?!_))+?)(\s*)\2|((?:[^_\n]|_(?!_))+?)(\s+)\2)(\S|$)?/g;
+const underscoreBoldRegex =
+  /([^_\s]+)?\s*(__)(?:\s+((?:__(?! )|_(?!_)|[^_\n])+?)(\s*)\2|((?:__(?! )|_(?!_)|[^_\n])+?)(\s+)\2)(\S|$)?/g;
 
 // Pattern for * italic *
 const asteriskItalicRegex = /([^*\s]+)?\s*(\*)(?!\*)(?:\s+([^*\n]+?)(\s*)\2|([^*\n]+?)(\s+)\2)(\S|$)?/g;
 
 // Pattern for _ italic _
-const underscoreItalicRegex = /([^_\s]+)?\s*(_)(?!_)(?:\s+([^_\n]+?)(\s*)\2|([^_\n]+?)(\s+)\2)(\S|$)?/g;
+const underscoreItalicRegex =
+  /([^_\s]+)?\s*(_)(?!_)(?:\s+((?:[^_\n]|_(?! ))+?)(\s*)\2|((?:[^_\n]|_(?! ))+?)(\s+)\2)(\S|$)?/g;
 
 // CommonMark ignores intraword underscores or asteriks, but we want to italicize/bold the inner part
 // Pattern for intraword _word_ in words like hello_world_
