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, 6 insertions, 1 deletions

diff --git a/regress/roff/ds/Makefile b/regress/roff/ds/Makefile
index 773105e8..68a8b3bf 100644
--- a/regress/roff/ds/Makefile
+++ b/regress/roff/ds/Makefile
@@ -1,4 +1,9 @@
-# $OpenBSD: Makefile,v 1.6 2019/02/06 20:54:28 schwarze Exp $
+# $OpenBSD: Makefile,v 1.7 2022/06/03 11:50:25 schwarze Exp $
+#
+# This directory is intended for tests of string *definitions*,
+# in particular testing the behaviour of the .ds and .as macros.
+# Tests of string *expansion* are better placed in the roff/string
+# directory.
 
 REGRESS_TARGETS = append escname nested quoting tab