Help:ENV: note escaping needed for special characters

Help/variable/ENV.rst

@@ -8,6 +8,17 @@ Use the syntax ``$ENV{VAR}`` to read environment variable ``VAR``.
 To test whether an environment variable is defined, use the signature
 ``if(DEFINED ENV{<name>})`` of the :command:`if` command.
 
+.. note::
+
+  Environment variable names containing special characters like parentheses
+  may need to be escaped.  (Policy :policy:`CMP0053` must also be enabled.)
+  For example, to get the value of the Windows environment variable
+  ``ProgramFiles(x86)``, use:
+
+  .. code-block:: cmake
+
+      set(ProgramFiles_x86 "$ENV{ProgramFiles\(x86\)}")
+
 For general information on environment variables, see the
 :ref:`Environment Variables <CMake Language Environment Variables>`
 section in the :manual:`cmake-language(7)` manual.