topic_link_util: Add [ and ] as characters to escape.

These characters cause the fallback markdown
links produced (#30071) to be broken.

Broken links are not produced when these are present
in `#**channel>topic**` syntax. It is only a problem
with the fallback markdown links.

web/src/topic_link_util.ts

@@ -4,7 +4,7 @@ import * as internal_url from "../shared/src/internal_url.ts";
 
 import * as stream_data from "./stream_data.ts";
 
-const invalid_stream_topic_regex = /[`>*&]|(\$\$)/g;
+const invalid_stream_topic_regex = /[`>*&[\]]|(\$\$)/g;
 
 export function will_produce_broken_stream_topic_link(word: string): boolean {
     return invalid_stream_topic_regex.test(word);
@@ -22,6 +22,10 @@ export function escape_invalid_stream_topic_characters(text: string): string {
             return "&";
         case "$$":
             return "$$";
+        case "[":
+            return "[";
+        case "]":
+            return "]";
         default:
             return text;
     }

web/tests/topic_link_util.test.cjs

@@ -32,9 +32,18 @@ const dollar_stream = {
     type: "stream",
 };
 
+const markdown_stream = {
+    name: "Markdown [md]",
+    description: "markdown",
+    stream_id: 7,
+    subscribed: true,
+    type: "stream",
+};
+
 stream_data.add_sub(sweden_stream);
 stream_data.add_sub(denmark_stream);
 stream_data.add_sub(dollar_stream);
+stream_data.add_sub(markdown_stream);
 
 run_test("stream_topic_link_syntax_test", () => {
     assert.equal(
@@ -78,11 +87,24 @@ run_test("stream_topic_link_syntax_test", () => {
         topic_link_util.get_fallback_markdown_link("$$MONEY$$"),
         "[#$$MONEY$$](#narrow/channel/6-.24.24MONEY.24.24)",
     );
+    assert.equal(
+        topic_link_util.get_fallback_markdown_link("Markdown [md]"),
+        "[#Markdown [md]](#narrow/channel/7-Markdown-.5Bmd.5D)",
+    );
 
     assert.equal(
         topic_link_util.get_stream_topic_link_syntax("Sweden", "&ab"),
         "[#Sweden > &ab](#narrow/channel/1-Sweden/topic/.26ab)",
     );
+    assert.equal(
+        topic_link_util.get_stream_topic_link_syntax("Sweden", "&ab]"),
+        "[#Sweden > &ab]](#narrow/channel/1-Sweden/topic/.26ab.5D)",
+    );
+
+    assert.equal(
+        topic_link_util.get_stream_topic_link_syntax("Sweden", "&a[b"),
+        "[#Sweden > &a[b](#narrow/channel/1-Sweden/topic/.26a.5Bb)",
+    );
 
     // Only for full coverage of the module.
     assert.equal(topic_link_util.escape_invalid_stream_topic_characters("Sweden"), "Sweden");