During identifier parsing, handle undefined escape sequences

in the same way as groff:
* \\ is always reduced to \
* \. is always reduced to .
* other undefined escape sequences are usually reduced to the escape name,
for example \G to G, except during the expansion of expanding escape
sequences having the standard argument form (in particular \* and \n),
in which case the backslash is preserved literally.

Yes, this is confusing indeed.
For example, the following have the same meaning:
* .ds \.   and   .ds .     which is not the same as   .ds \\.
* \*[\.]   and   \*[.]     which is not the same as   \*[\\.]
* .ds \G   and   .ds G     which is not the same as   .ds \\G
* \*[\G]   and   \*[\\G]   which is not the same as   \*[G]   <- sic!

To feel less dirty, have a leaning toothpick, if you are so inclined.

This patch also slightly improves the string shown by the "escaped
character not allowed in a name" error message.

1 files changed, 2 insertions, 2 deletions

diff --git a/regress/roff/nr/escname.out_ascii b/regress/roff/nr/escname.out_ascii
index ca2d50f5..d0301af9 100644
--- a/regress/roff/nr/escname.out_ascii
+++ b/regress/roff/nr/escname.out_ascii
@@ -4,7 +4,7 @@ NNAAMMEE
        nr-escname - escape sequences in register names
 
 DDEESSCCRRIIPPTTIIOONN
-       1 2 3
+       1 2 3 5 6
 
        0 2 3
 
@@ -12,4 +12,4 @@ DDEESSCCRRIIPPTTIIOONN
 
        incomplete:
 
-OpenBSD                          June 29, 2014                   NR-ESCNAME(1)
+OpenBSD                          June 3, 2022                    NR-ESCNAME(1)