GNU ed(1) is not strictly POSIX compliant, as described in the
POSIX 1003.2-1992 document.  The following is a summary of omissions
and extensions to, and deviations from, the POSIX standard.

OMISSIONS
---------
  * Locale(3) is not supported yet.

EXTENSIONS
----------
  * Though GNU ed is not a stream editor, it can be used to edit binary files.
    To assist in binary editing, when a file containing at least one ASCII
    NUL character is written, a newline is not appended if it did not
    already contain one upon reading.  In particular, reading /dev/null
    prior to writing prevents appending a newline to a binary file.

    For example, to create a file with GNU ed containing a single NUL character:
      $ ed file
      a
      ^@
      .
      r /dev/null
      wq

    Similarly, to remove a newline from the end of binary `file':
      $ ed file
      r /dev/null
      wq

  * BSD commands have been implemented wherever they do not conflict with
    the POSIX standard.  The BSD-ism's included are:
      * `s' (i.e., s[n][rgp]*) to repeat a previous substitution,
      * `W' for appending text to an existing file,
      * `wq' for exiting after a write,
      * `z' for scrolling through the buffer, and
      * BSD line-addressing syntax (i.e., `^' and `%')  is recognized.

  * The POSIX interactive global commands `G' and `V' are extended to
    support multiple commands, including `a', `i' and `c'.  The command
    format is the same as for the global commands `g' and `v', i.e., one
    command per line with each line, except for the last, ending in a
    backslash (\).

  * The file commands `E', `e', `r', `W' and `w'  process a <file>
    argument for backslash escapes; i.e.,  any character preceded by a
    backslash is interpreted literally.  If the first unescaped character
    of a <file> argument is a bang (!), then the rest of the line is
    interpreted as a shell command, and no escape processing is performed
    by GNU ed.

  * For SunOS ed(1) compatibility, GNU ed runs in restricted mode if invoked
    as red.  This limits editing of files in the local directory only and
    prohibits shell commands.

DEVIATIONS
----------
  * For backwards compatibility, the POSIX rule that says a range of
    addresses cannot be used where only a single address is expected has
    been relaxed.

  * To support the BSD `s' command (see EXTENSIONS above),
    substitution patterns cannot be delimited by numbers or the characters
    `r', `g' and `p'.  In contrast, POSIX specifies any character expect
    space or newline can used as a delimiter.

  * Since the behavior of `u' (undo) within a `g' (global) command list is
    not specified by POSIX, GNU ed follows the behavior of the SunOS ed:
    undo forces a global command list to be executed only once, rather than
    for each line matching a global pattern.  In addtion, each instance of
    `u' within a global command undoes all previous commands (including
    undo's) in the command list.  This seems the best way, since the
    alternatives are either too complicated to implement or too confusing
    to use.

  * The `m' (move) command within a `g' command list also follows the SunOS
    ed implementation: any moved lines are removed from the global command's
    `active' list.

  * If GNU ed is invoked with a name argument prefixed by a bang (!), then
    the remainder of the argument is interpreted as a shell command.  To invoke
    ed on a file whose name starts with bang, prefix the name with a
    (quoted) backslash.

  * For backwards compatibility, errors in piped scripts do not force ed
    to exit.  POSIX only specifies ed's response for input via regular
    files (including here documents) or tty's.