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, 13 insertions, 2 deletions

diff --git a/regress/roff/de/escname.out_ascii b/regress/roff/de/escname.out_ascii
index 367b5d0f..e096fd71 100644
--- a/regress/roff/de/escname.out_ascii
+++ b/regress/roff/de/escname.out_ascii
@@ -12,7 +12,18 @@ DDEESSCCRRIIPPTTIIOONN
 
      define first = val1
 
-     Values (first, second, first\second): val1 val2 val3
+     define first\.second = val_dot
+
+     define first\Gsecond = val_inval
+
+     Values:
+     first          val1
+     second         val2
+     first\second   val3
+     first.second   val_dot
+     first\.second  val_dot
+     firstGsecond   val_inval
+     first\Gsecond  val_inval
 
      Remove all but second: val2
 
@@ -20,4 +31,4 @@ DDEESSCCRRIIPPTTIIOONN
 
      final text
 
-OpenBSD                          July 4, 2017                          OpenBSD
+OpenBSD                          June 3, 2022                          OpenBSD