fix: Loosen italic markdown matching a little

see: https://github.com/outline/outline/discussions/3336

shared/editor/marks/Italic.ts

@@ -25,9 +25,29 @@ export default class Italic extends Mark {
   }
 
   inputRules({ type }: { type: MarkType }): InputRule[] {
+    /**
+     * Due to use of snake_case strings common in docs the matching conditions
+     * are a bit more strict than e.g. the ** bold syntax to help prevent
+     * false positives.
+     *
+     * Matches:
+     * _1_
+     * _123_
+     * (_one_
+     * [_one_
+     *
+     * No match:
+     * __
+     * __123_
+     * __123__
+     * _123
+     * one_123_
+     * ONE_123_
+     * 1_123_
+     */
     return [
-      markInputRule(/(?:^|[\s])(_([^_]+)_)$/, type),
-      markInputRule(/(?:^|[^*])(\*([^*]+)\*)$/, type),
+      markInputRule(/(?<=[^_\na-zA-Z0-9]|^)_([^_\n]+)_$/, type),
+      markInputRule(/(?<=[^*\na-zA-Z0-9]|^)\*([^*\n]+)\*$/, type),
     ];
   }