Add special lexing for regexes after `=~`

corpus/expressions.txt

@@ -18,22 +18,35 @@ Words with special characters
 echo {o[k]}
 echo }}}
 echo ]]] ===
-[[ "35d8b" =~ ^[0-9a-fA-F] ]] || echo {nomatch}
 
 ---
 
 (program
   (command (command_name (word)) (concatenation (word) (word)))
   (command (command_name (word)) (word))
-  (command (command_name (word)) (word) (word))
-  (list
+  (command (command_name (word)) (word) (word)))
+
+=============================
+The regex operator
+=============================
+
+[[ "35d8b" =~ ^[0-9a-fA-F] ]]
+[[ $CMD =~ (^|;)update_terminal_cwd($|;) ]]
+[[ ! " ${completions[*]} " =~ " $alias_cmd " ]]
+
+---
+
+(program
   (bracket_command
     (string)
+    (regex))
+  (bracket_command
+    (simple_expansion (variable_name))
+    (regex))
+  (bracket_command
     (word)
-      (concatenation (word) (word)))
-    (command
-      (command_name (word))
-      (concatenation (word)))))
+    (string (expansion (subscript (variable_name) (word))))
+    (string (simple_expansion (variable_name)))))
 
 =============================
 Simple variable expansions

grammar.js

@@ -179,10 +179,20 @@ module.exports = grammar({
       $._statement
     )),
 
-    bracket_command: $ => choice(
-      seq('[', repeat1($._expression), ']'),
-      seq('[[', repeat1($._expression), ']]')
-    ),
+    bracket_command: $ => {
+      const contents = repeat1(choice(
+        $._expression,
+        seq('=~', choice(
+          $.regex,
+          $._expression
+        ))
+      ))
+
+      return choice(
+        seq('[', contents, ']'),
+        seq('[[', contents, ']]')
+      )
+    },
 
     // Commands
 
@@ -383,6 +393,8 @@ module.exports = grammar({
       seq('\\', noneOf('\\s'))
     ))),
 
+    regex: $ => /\S+/,
+
     _terminator: $ => choice(';', ';;', '\n', '&')
   }
 });