Comments apply to http://www.tcl.tk/man/tcl8.5/tutorial/Tcl21.html on 2008-12-24.
The lesson deals with having regular expressions quoted rather braced, and thus subjected to substitution. OK, this leads to a bit of Quoting Hell as the title suggests, but there's little reason to go there anymore.
In the Tcl 7 days you had to have Tcl do substitution of \t, \n, etc. because the old regexp engine didn't do these, but the current (Tcl 8.1+) engine does, so the bullet point
* A backslash sequence (\n, \t, etc) has meaning to the Tcl substitution phase, but not to the regular expression parser.
is simply wrong, This means at least the "Extracting a hexadecimal value" example should be changed.