diff --git a/corpus/arguments.txt b/corpus/argument.txt
similarity index 98%
rename from corpus/arguments.txt
rename to corpus/argument.txt
index 0abb389..87b293d 100644
--- a/corpus/arguments.txt
+++ b/corpus/argument.txt
@@ -117,10 +117,8 @@ nvim_buf_detach_event[{buf}]
       (word))
     (line
       (word)
-      (word)
-      (argument
-        (word))
-      (word))))
+      (optional_arg
+        (word)))))
 
 ================================================================================
 NOT an argument
diff --git a/corpus/argument_optional.txt b/corpus/argument_optional.txt
new file mode 100644
index 0000000..fb1a649
--- /dev/null
+++ b/corpus/argument_optional.txt
@@ -0,0 +1,40 @@
+================================================================================
+optional [argument]
+================================================================================
+:ar[gs]! [++opt] [+cmd] {arglist}			*:args_f!*
+:[count]arge[dit][!] [++opt] [+cmd] {name} ..
+
+
+--------------------------------------------------------------------------------
+
+(help_file
+  (block
+    (line
+      (word)
+      (optional_arg
+        (word))
+      (word)
+      (optional_arg
+        (word))
+      (optional_arg
+        (word))
+      (argument
+        (word))
+      (tag
+        (word)))
+    (line
+      (word)
+      (optional_arg
+        (word))
+      (word)
+      (optional_arg
+        (word))
+      (optional_arg
+        (word))
+      (optional_arg
+        (word))
+      (optional_arg
+        (word))
+      (argument
+        (word))
+      (word))))
diff --git a/corpus/codeblock.txt b/corpus/codeblock.txt
index 0a9c9a9..a138078 100644
--- a/corpus/codeblock.txt
+++ b/corpus/codeblock.txt
@@ -398,8 +398,8 @@ codeblock stop and start on same line
     (line
       (word)
       (word)
-      (word)
-      (word))
+      (optional_arg
+        (word)))
     (line
       (argument
         (word))))
diff --git a/corpus/taglink.txt b/corpus/taglink.txt
index 894eeb3..8bdea96 100644
--- a/corpus/taglink.txt
+++ b/corpus/taglink.txt
@@ -56,8 +56,8 @@ Hello |world| hello
       (taglink
         (word))
       (word)
-      (word)
-      (word)
+      (optional_arg
+        (word))
       (word))
     (line
       (taglink
diff --git a/corpus/url.txt b/corpus/url.txt
index ea53720..8e7f4b1 100644
--- a/corpus/url.txt
+++ b/corpus/url.txt
@@ -54,11 +54,9 @@ markdown: [https://neovim.io/doc/user/#yay](https://neovim.io/doc/user/#yay).
       (word))
     (line
       (word)
-      (word)
-      (url
+      (optional_arg
         (word))
       (word)
-      (word)
       (url
         (word))
       (word))))
diff --git a/grammar.js b/grammar.js
index aff66e2..ff13d76 100644
--- a/grammar.js
+++ b/grammar.js
@@ -53,6 +53,7 @@ module.exports = grammar({
         $.taglink,
         $.codespan,
         $.argument,
+        $.optional_arg,
         $.keycode,
       ),
 
@@ -210,6 +211,8 @@ module.exports = grammar({
     codespan: ($) => _word($, /[^``\n]+/, '`', '`'),
     // Argument: {arg} (no whitespace allowed)
     argument: ($) => _word($, /[^}\n\t ]+/, '{', '}'),
+    // Optional argument: [arg] (no whitespace allowed)
+    optional_arg: ($) => _word($, /[^\]\n\t ]+/, '[', ']'),
   },
 });