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+The formal syntax, as specified using
+`BNF <https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form>`_ notation with
+the regular extensions, for registry query is the following:
+
+.. parsed-literal::
+
+  registry_query  ::= '[' `sep_definition`_? `root_key`_
+                      ((`key_separator`_ `sub_key`_)? (`value_separator`_ `value_name`_)?)? ']'
+  _`sep_definition`  ::= '{' `value_separator`_ '}'
+  _`root_key`        ::= 'HKLM' | 'HKEY_LOCAL_MACHINE' | 'HKCU' | 'HKEY_CURRENT_USER' |
+                      'HKCR' | 'HKEY_CLASSES_ROOT' | 'HKCC' | 'HKEY_CURRENT_CONFIG' |
+                      'HKU' | 'HKEY_USERS'
+  _`sub_key`         ::= `element`_ (`key_separator`_ `element`_)*
+  _`key_separator`   ::= '/' | '\\'
+  _`value_separator` ::= `element`_ | ';'
+  _`value_name`      ::= `element`_ | '(default)'
+  _`element`         ::= `character`_\+
+  _`character`       ::= <any character except `key_separator`_ and `value_separator`_>
+
+The `sep_definition`_ optional item offers the possibility to specify the
+string used to separate the `sub_key`_ from the `value_name`_ item. If
+not specified, the character ``;`` is used.
+
+.. parsed-literal::
+
+  # example using default separator
+  |FIND_XXX| (... **PATHS** "/root/[HKLM/Stuff;InstallDir]/lib[HKLM\\\\Stuff;Architecture]")
+
+  # example using different specified separators
+  |FIND_XXX| (... **HINTS** "/root/[{|}HKCU/Stuff|InstallDir]/lib[{@@}HKCU\\\\Stuff@@Architecture]")
+
+If the `value_name`_ item is not specified or has the special name
+``(default)``, the content of the default value, if any, will be returned. The
+supported types for the `value_name`_ are:
+
+* ``REG_SZ``.
+* ``REG_EXPAND_SZ``. The returned data is expanded.
+* ``REG_DWORD``.
+* ``REG_QWORD``.
+
+When the registry query failed, typically because the key does not exist or
+the data type is not supported, the string ``/REGISTRY-NOTFOUND`` is substituted
+to the ``[]`` query expression.