The purpose of the rules file is to map between configuration values that are easy for a user to specify and understand, and the configuration values xkbcomp uses and understands.

xkbcomp uses the xkb_component_names struct, which maps directly to include statements of the appropriate sections, called for short KcCGST (see the XKB introduction; 'G' stands for "geometry", which is not supported). These are not really intuitive nor straightforward for the uninitiated.

Instead, the user passes in a xkb_rule_names struct, which consists of the name of a rules file (in Linux this is usually "evdev"), a keyboard model (e.g. "pc105"), a set of layouts (which will end up in different groups, e.g. "us,fr"), variants (used to alter/augment the respective layout, e.g. "intl,dvorak"), and a set of options (used to tweak some general behavior of the keyboard, e.g. "ctrl:nocaps,compose:menu" to make the Caps Lock key act like Ctrl and the Menu key like Compose). We call these RMLVO.

Format of the file

The file consists of rule sets, each consisting of rules (one per line), which match the MLVO values on the left hand side, and, if the values match to the values the user passed in, results in the values on the right hand side being added to the resulting KcCGST. Since some values are related and repeated often, it is possible to group them together and refer to them by a group name in the rules.

Along with matching values by simple string equality, and for membership in a group defined previously, rules may also contain "wildcard" values - "*" - which always match. These usually appear near the end.

Grammar

(It might be helpful to look at a file like rules/evdev along with this grammar. Comments, whitespace, etc. are not shown.)

File         ::= { "!" (Include | Group | RuleSet) }
 
Include      ::= "include" <ident>
 
Group        ::= GroupName "=" { GroupElement } "\n"
GroupName    ::= "$"<ident>
GroupElement ::= <ident>
 
RuleSet      ::= Mapping { Rule }
 
Mapping      ::= { Mlvo } "=" { Kccgst } "\n"
Mlvo         ::= "model" | "option" | ("layout" | "variant") [ Index ]
Index        ::= "[" 1..XKB_NUM_GROUPS "]"
Kccgst       ::= "keycodes" | "symbols" | "types" | "compat" | "geometry"
 
Rule         ::= { MlvoValue } "=" { KccgstValue } "\n"
MlvoValue    ::= "*" | GroupName | <ident>
KccgstValue  ::= <ident>

Notes:

Include processes the rules in the file path specified in the ident, in order. %-expansion is performed, as follows:

%%:
  A literal %.
 
%H:
  The value of the HOME environment variable.
 
%E:
  The extra lookup path for system-wide XKB data (usually /etc/xkb/rules).
 
%S:
  The system-installed rules directory (usually /usr/share/X11/xkb/rules).

The order of values in a Rule must be the same as the Mapping it follows. The mapping line determines the meaning of the values in the rules which follow in the RuleSet.
If a Rule is matched, %-expansion is performed on the KccgstValue, as follows:

%m, %l, %v:
   The model, layout or variant, if only one was given (e.g.
   %l for "us,il" is invalid).
 
%l[1], %v[1]:
   Layout or variant for the specified group Index, if more than
   one was given (e.g. %l[1] for "us" is invalid).
 
%+m, %+l, %+v, %+l[1], %+v[1]
   As above, but prefixed with '+'. Similarly, '|', '-', '_' may be
   used instead of '+'.
 
%(m), %(l), %(l[1]), %(v), %(v[1]):
   As above, but prefixed by '(' and suffixed by ')'.

In case the expansion is invalid, as described above, it is skipped (the rest of the string is still processed); this includes the prefix and suffix (that's why you shouldn't use e.g. "(%v[1])").