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, 7 insertions, 1 deletions

diff --git a/regress/roff/cond/string.out_ascii b/regress/roff/cond/string.out_ascii
index 2d80a903..c67c0c55 100644
--- a/regress/roff/cond/string.out_ascii
+++ b/regress/roff/cond/string.out_ascii
@@ -19,4 +19,10 @@ DDEESSCCRRIIPPTTIIOONN
 
        escape sequence after identifier: -mystr is defined
 
-OpenBSD                        February 6, 2019                      STRING(1)
+       backslash in name: \ is defined
+
+       dot in name: . is defined \. is defined
+
+       invalid escape in name: G is defined \G is defined
+
+OpenBSD                          June 3, 2022                        STRING(1)