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, 8 insertions, 2 deletions

diff --git a/regress/roff/cond/register.out_ascii b/regress/roff/cond/register.out_ascii
index 928c6138..673b9f12 100644
--- a/regress/roff/cond/register.out_ascii
+++ b/regress/roff/cond/register.out_ascii
@@ -7,8 +7,14 @@ DDEESSCCRRIIPPTTIIOONN
        not yet defined
        now defined
 
-       identifier + identifier:      myreg is defined
+       tab after identifier:    myreg is defined
 
        escape sequence after identifier: -myreg is defined
 
-OpenBSD                        February 6, 2019                    REGISTER(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                      REGISTER(1)