[FileCheck] doc update/fix

[SjoerdMeijer]

Summary

This adds to the documentation that the matcher supports/implements POSIX BRE and ERE.
It also fixes a minor issue in the regexp describing a variable name: underscores are allowed too.

[SjoerdMeijer]

I would like to add links for BRE and ERE, to get a quick syntax overview. Not sure what page would be best for that, and if we e.g. can or like to link to wikipedia (https://en.wikipedia.org/wiki/Regular_expression#POSIX_basic_and_extended) or something else.

[bkramer]

Does FileCheck actually support BRE? I thought it's always ERE.

[SjoerdMeijer]

Thanks for reviewing/checking. The regexp class implements both BRE and ERE, but it defaults to using ERE. Thus FileCheck defaults to ERE. As the mode is not exposed to the user (with e.g. an option), I kept it simple and now say that FileCheck supports ERE.

[bkramer]

lg

llvm/trunk/docs/CommandGuide/FileCheck.rst

 
 FileCheck Pattern Matching Syntax
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 All FileCheck directives take a pattern to match.
 For most uses of FileCheck, fixed string matching is perfectly sufficient.  For
 some things, a more flexible form of matching is desired.  To support this,
 FileCheck allows you to specify regular expressions in matching strings,
-surrounded by double braces: ``{{yourregex}}``.  Because we want to use fixed
-string matching for a majority of what we do, FileCheck has been designed to
-support mixing and matching fixed string matching with regular expressions.
-This allows you to write things like this:
+surrounded by double braces: ``{{yourregex}}``. FileCheck implements a POSIX
+regular expression matcher; it supports Extended POSIX regular expressions
+(ERE). Because we want to use fixed string matching for a majority of what we
+do, FileCheck has been designed to support mixing and matching fixed string
+matching with regular expressions.  This allows you to write things like this:
 
 .. code-block:: llvm
 
    ; CHECK: movhpd  {{[0-9]+}}(%esp), {{%xmm[0-7]}}
 
 In this case, any offset from the ESP register will be allowed, and any xmm
 register will be allowed.
 
.. code-block:: llvm
    ; CHECK: test5:
    ; CHECK:    notw [[REGISTER:%[a-z]+]]
    ; CHECK:    andw {{.*}}[[REGISTER]]
 
 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,
+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