Re: [Groff] [Groff & Heirloom] tbl problem with backslashes

[Ralph Corderoy]

Hi,

Ted wrote:
> WHile not quite sure about the details of the logic underlying the
> effect you describe

It's quite involved, as I recall.  This bit of groff's manual seems
relevant.

    $ info groff 2>/dev/null | sed -n '/-- Escape: \\\\/,/=> 1/p' | cat -s
     -- Escape: \\
     -- Escape: \e
     -- Escape: \E
         Print the current escape character (which is the backslash
         character `\' by default).

         `\\' is a `delayed' backslash; more precisely, it is the default
         escape character followed by a backslash, which no longer has
         special meaning due to the leading escape character.  It is _not_
         an escape sequence in the usual sense!  In any unknown escape
         sequence `\X' the escape character is ignored and X is printed.
         But if X is equal to the current escape character, no warning is
         emitted.

         As a consequence, only at top-level or in a diversion a backslash
         glyph is printed; in copy-in mode, it expands to a single
         backslash which then combines with the following character to an
         escape sequence.

         The `\E' escape differs from `\e' by printing an escape character
         that is not interpreted in copy mode.  Use this to define strings
         with escapes that work when used in copy mode (for example, as a
         macro argument).  The following example defines strings to begin
         and end a superscript:

              .ds { \v'-.3m'\s'\En[.s]*60/100'
              .ds } \s0\v'.3m'

         Another example to demonstrate the differences between the various
         escape sequences, using a strange escape character, `-'.

              .ec -
              .de xxx
              --A'123'
              ..
              .xxx
                  => -A'foo'

         The result is surprising for most users, expecting `1' since `foo'
         is a valid identifier.  What has happened?  As mentioned above,
         the leading escape character makes the following character
         ordinary.  Written with the default escape character the sequence
         `--' becomes `\-' - this is the minus sign.

         If the escape character followed by itself is a valid escape
         sequence, only `\E' yields the expected result:

              .ec -
              .de xxx
              -EA'123'
              ..
              .xxx
                  => 1
    $

Cheers, Ralph.