Help: Document CMAKE_MATCH_<n> variables

Help/command/if.rst

@@ -104,6 +104,7 @@ Possible expressions are:
 ``if(<variable|string> MATCHES regex)``
  True if the given string or variable's value matches the given regular
  expression.  See :ref:`Regex Specification` for regex format.
+ ``()`` groups are captured in :variable:`CMAKE_MATCH_<n>` variables.
 
 ``if(<variable|string> LESS <variable|string>)``
  True if the given string or variable's value is a valid number and less

Help/command/string.rst

@@ -113,7 +113,7 @@ The following characters have special meaning in regular expressions:
   in the ``REGEX REPLACE`` operation. Additionally it is saved
   by all regular expression-related commands, including
   e.g. :command:`if(MATCHES)`, in the variables
-  ``CMAKE_MATCH_<n>`` for ``<n>`` 0..9.
+  :variable:`CMAKE_MATCH_<n>` for ``<n>`` 0..9.
 
 ``*``, ``+`` and ``?`` have higher precedence than concatenation.  ``|``
 has lower precedence than concatenation.  This means that the regular

Help/manual/cmake-variables.7.rst

@@ -56,6 +56,7 @@ Variables that Provide Information
    /variable/CMAKE_MAJOR_VERSION
    /variable/CMAKE_MAKE_PROGRAM
    /variable/CMAKE_MATCH_COUNT
+   /variable/CMAKE_MATCH_n
    /variable/CMAKE_MINIMUM_REQUIRED_VERSION
    /variable/CMAKE_MINOR_VERSION
    /variable/CMAKE_PARENT_LIST_FILE

Help/variable/CMAKE_MATCH_COUNT.rst

@@ -3,6 +3,7 @@ CMAKE_MATCH_COUNT
 
 The number of matches with the last regular expression.
 
-When a regular expression match is used, CMake fills in ``CMAKE_MATCH_<n>``
-variables with the match contents. The ``CMAKE_MATCH_COUNT`` variable holds
-the number of match expressions when these are filled.
+When a regular expression match is used, CMake fills in
+:variable:`CMAKE_MATCH_<n>` variables with the match contents.
+The ``CMAKE_MATCH_COUNT`` variable holds the number of match
+expressions when these are filled.

Help/variable/CMAKE_MATCH_n.rst

@@ -0,0 +1,10 @@
+CMAKE_MATCH_<n>
+---------------
+
+Capture group ``<n>`` matched by the last regular expression, for groups
+0 through 9.  Group 0 is the entire match.  Groups 1 through 9 are the
+subexpressions captured by ``()`` syntax.
+
+When a regular expression match is used, CMake fills in ``CMAKE_MATCH_<n>``
+variables with the match contents.  The :variable:`CMAKE_MATCH_COUNT`
+variable holds the number of match expressions when these are filled.