[optional]

Problem:
Using _word() to define an optional arg leads to a lot of syntax errors in
real-world :help files because [brackets] are commonly used for non-args.

Solution:
Define (optional) with a regex, like (keycode) instead of using the more
"formal" mechanism. Regex has "best-effort" behavior, while seq() behaves more
strictly.

Don't treat `[{arg}]` as (optional), else it clobbers the nested `{arg}`.

fix #1

corpus/argument.txt

@@ -117,8 +117,10 @@ nvim_buf_detach_event[{buf}]
       (word))
     (line
       (word)
-      (optional_arg
-        (word)))))
+      (word)
+      (argument
+        (word))
+      (word))))
 
 ================================================================================
 NOT an argument

corpus/argument_optional.txt

@@ -11,30 +11,22 @@ optional [argument]
   (block
     (line
       (word)
-      (optional_arg
-        (word))
+      (optional)
       (word)
-      (optional_arg
-        (word))
-      (optional_arg
-        (word))
+      (optional)
+      (optional)
       (argument
         (word))
       (tag
         (word)))
     (line
       (word)
-      (optional_arg
-        (word))
+      (optional)
       (word)
-      (optional_arg
-        (word))
-      (optional_arg
-        (word))
-      (optional_arg
-        (word))
-      (optional_arg
-        (word))
+      (optional)
+      (optional)
+      (optional)
+      (optional)
       (argument
         (word))
       (word))))

corpus/codeblock.txt

@@ -398,8 +398,7 @@ codeblock stop and start on same line
     (line
       (word)
       (word)
-      (optional_arg
-        (word)))
+      (optional))
     (line
       (argument
         (word))))

corpus/taglink.txt

@@ -56,8 +56,7 @@ Hello |world| hello
       (taglink
         (word))
       (word)
-      (optional_arg
-        (word))
+      (optional)
       (word))
     (line
       (taglink

corpus/url.txt

@@ -54,8 +54,7 @@ markdown: [https://neovim.io/doc/user/#yay](https://neovim.io/doc/user/#yay).
       (word))
     (line
       (word)
-      (optional_arg
-        (word))
+      (optional)
       (word)
       (url
         (word))

grammar.js

@@ -53,7 +53,7 @@ module.exports = grammar({
         $.taglink,
         $.codespan,
         $.argument,
-        $.optional_arg,
+        $.optional,
         $.keycode,
       ),
 
@@ -96,6 +96,9 @@ module.exports = grammar({
       /ALT-./,
     ),
 
+    // Optional argument: [arg] (no whitespace allowed)
+    optional: () => /\[[^\]{\n\t ]+\]/,
+
     // First part (minus tags) of h3 or column_heading.
     uppercase_name: () => seq(
       token.immediate(_uppercase_word),  // No whitespace before heading.
@@ -211,8 +214,6 @@ 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 ]+/, '[', ']'),
   },
 });