+The first check line matches a regex ``%[a-z]+`` and captures it into the
+variable ``REGISTER``. The second line verifies that whatever is in
+``REGISTER`` occurs later in the file after an "``andw``". :program:`FileCheck`
+variable references are always contained in ``[[ ]]`` pairs, and their names can
+be formed with the regex ``[a-zA-Z][a-zA-Z0-9]*``. If a colon follows the name,
+then it is a definition of the variable; otherwise, it is a use.
+
+:program:`FileCheck` variables can be defined multiple times, and uses always
+get the latest value. Variables can also be used later on the same line they
+were defined on. For example:
+
+.. code-block:: llvm
+
+ ; CHECK: op [[REG:r[0-9]+]], [[REG]]
+
+Can be useful if you want the operands of ``op`` to be the same register,
+and don't care exactly which register it is.
+
+FileCheck Expressions
+~~~~~~~~~~~~~~~~~~~~~
+
+Sometimes there's a need to verify output which refers line numbers of the
+match file, e.g. when testing compiler diagnostics. This introduces a certain
+fragility of the match file structure, as "``CHECK:``" lines contain absolute
+line numbers in the same file, which have to be updated whenever line numbers
+change due to text addition or deletion.
+
+To support this case, FileCheck allows using ``[[@LINE]]``,
+``[[@LINE+<offset>]]``, ``[[@LINE-<offset>]]`` expressions in patterns. These
+expressions expand to a number of the line where a pattern is located (with an
+optional integer offset).
+
+This way match patterns can be put near the relevant test lines and include
+relative line number references, for example:
+
+.. code-block:: c++
+
+ // CHECK: test.cpp:[[@LINE+4]]:6: error: expected ';' after top level declarator
+ // CHECK-NEXT: {{^int a}}
+ // CHECK-NEXT: {{^ \^}}
+ // CHECK-NEXT: {{^ ;}}
+ int a