const
CtrlC = ^C;
begin
Write(Ord(CtrlC));
end.This defines a Char constant with value #3, then writes 3 in Borland Pascal 7, and I remember seeing it years before that too.
I just checked the Turbo Pascal 5.0 and Borland Pascal 7.0 languages guides, but could not find it, so it seems undocumented.
Edit: I do remember this was a Borland thing, and just [WayBack] checked: it is not part of the ISO Pascal standard (formerly this was ANSI Pascal Standard, thanks Sertac for noticing this).
It [WayBack] is documented in the Free Pascal documentation [WayBack].
SGI uses the backslash as escape character, as per their docs [WayBack].
More Edit: I found it [WayBack] documented in Delphi in a Nutshell and the [WayBack] Delphi Basics site.
Found it: Just found it on page 37 of the Turbo Pascal 3 Reference Manual [WayBack].
The Wiert Corner - irregular stream of stuff 
Fred Ahrens said
I remember using this type of syntax heavily in Turbo Pascal 1 under CP/M for controlling the output on screen and to printers. It was the time when it was required to develop your own printer drivers for your software. And sometimes it was also required to have different screen drivers depending on the CP/M flavours you want to support with your software. Especially when you wanted to work with menus, windows and the sometimes already available text formatting features.