Syntax for Mercurial Ignore Files

    The Mercurial system uses a file called ``.hgignore`` in the root
    directory of a repository to control its behavior when it searches
    for files that it is not currently tracking.
    The working directory of a Mercurial repository will often contain
    files that should not be tracked by Mercurial. These include backup
    files created by editors and build products created by compilers.
    These files can be ignored by listing them in a ``.hgignore`` file in
    the root of the working directory. The ``.hgignore`` file must be
    created manually. It is typically put under version control, so that
    the settings will propagate to other repositories with push and pull.
    An untracked file is ignored if its path relative to the repository
    root directory, or any prefix path of that path, is matched against
    any pattern in ``.hgignore``.
    For example, say we have an untracked file, ``file.c``, at
    ``a/b/file.c`` inside our repository. Mercurial will ignore ``file.c``
    if any pattern in ``.hgignore`` matches ``a/b/file.c``, ``a/b`` or ``a``.
    In addition, a Mercurial configuration file can reference a set of
    per-user or global ignore files. See the ``ignore`` configuration
    key on the ``[ui]`` section of :hg:`help config` for details of how to
    configure these files.
    To control Mercurial's handling of files that it manages, many
    commands support the ``-I`` and ``-X`` options; see
    :hg:`help <command>` and :hg:`help patterns` for details.
    Files that are already tracked are not affected by .hgignore, even
    if they appear in .hgignore. An untracked file X can be explicitly
    added with :hg:`add X`, even if X would be excluded by a pattern
    in .hgignore.
    An ignore file is a plain text file consisting of a list of patterns,
    with one pattern per line. Empty lines are skipped. The ``#``
    character is treated as a comment character, and the ``\`` character
    is treated as an escape character.
    Mercurial supports several pattern syntaxes. The default syntax used
    is Python/Perl-style regular expressions.
    To change the syntax used, use a line of the following form::
      syntax: NAME
    where ``NAME`` is one of the following:
      Regular expression, Python/Perl syntax.
      Shell-style glob.
    The chosen syntax stays in effect when parsing all patterns that
    follow, until another syntax is selected.
    Neither glob nor regexp patterns are rooted. A glob-syntax pattern of
    the form ``*.c`` will match a file ending in ``.c`` in any directory,
    and a regexp pattern of the form ``\.c$`` will do the same. To root a
    regexp pattern, start it with ``^``.
    .. note::
      Patterns specified in other than ``.hgignore`` are always rooted.
      Please see :hg:`help patterns` for details.
    Here is an example ignore file. ::
      # use glob syntax.
      syntax: glob
      # switch to regexp syntax.
      syntax: regexp