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.

4 files changed, 39 insertions, 19 deletions

diff --git a/regress/roff/string/Makefile b/regress/roff/string/Makefile
index b8885c65..2fa10557 100644
--- a/regress/roff/string/Makefile
+++ b/regress/roff/string/Makefile
@@ -1,4 +1,9 @@
-# $OpenBSD: Makefile,v 1.6 2014/07/06 19:08:57 schwarze Exp $
+# $OpenBSD: Makefile,v 1.10 2022/06/03 11:50:25 schwarze Exp $
+#
+# This directory is intended for tests of string *expansion*,
+# in particular testing the behaviour of the \* escape sequence.
+# Tests of string *definitions* are better placed in the roff/ds
+# directory.
 
 REGRESS_TARGETS	 = dotT escape infinite name std undef zerolength
 LINT_TARGETS	 = name std undef
diff --git a/regress/roff/string/name.in b/regress/roff/string/name.in
index 2948f164..b2612be2 100644
--- a/regress/roff/string/name.in
+++ b/regress/roff/string/name.in
@@ -1,5 +1,5 @@
-.\" $OpenBSD: name.in,v 1.4 2017/07/04 14:53:27 schwarze Exp $
-.Dd $Mdocdate: July 4 2017 $
+.\" $OpenBSD: name.in,v 1.5 2022/06/03 11:50:25 schwarze Exp $
+.Dd $Mdocdate: June 3 2022 $
 .Dt STRING-NAME 1
 .Os
 .Sh NAME
@@ -10,7 +10,9 @@
 .ds "quot" value of "quot"
 .ds bs\e value of bs\ee
 .ds bs\\e value of bs\e\ee
+.ds dot. value of dot.
 .ds bl\ e value of bl\e e
+.ds inval\\G value of inval\eG
 norm: \*[norm]
 .br
 norm without closing brace: \*[norm
@@ -26,6 +28,10 @@ bs\e\ee: \*[bs\\e]
 bse: \*[bse]
 .br
 bs: \*[bs]
+.br
+dot.: \*[dot.]
+.br
+dot\e.: \*[dot\.]
 .\".br
 .\"bl\e e: \*[bl\ e]
 .br
@@ -34,3 +40,7 @@ bl e: \*[bl e]
 ble: \*[ble]
 .br
 bl: \*[bl]
+.br
+inval\e\eG: \*[inval\\G]
+.br
+inval\eG: \*[inval\G]
diff --git a/regress/roff/string/name.out_ascii b/regress/roff/string/name.out_ascii
index 325e28aa..38258a72 100644
--- a/regress/roff/string/name.out_ascii
+++ b/regress/roff/string/name.out_ascii
@@ -11,8 +11,12 @@ DDEESSCCRRIIPPTTIIOONN
      bs\\e: value of bs\\e
      bse:
      bs:
+     dot.: value of dot.
+     dot\.: value of dot.
      bl e:
      ble:
      bl:
+     inval\\G: value of inval\G
+     inval\G: value of inval\G
 
-OpenBSD                          July 4, 2017                          OpenBSD
+OpenBSD                          June 3, 2022                          OpenBSD
diff --git a/regress/roff/string/name.out_lint b/regress/roff/string/name.out_lint
index 5a6340a0..13283f5d 100644
--- a/regress/roff/string/name.out_lint
+++ b/regress/roff/string/name.out_lint
@@ -1,16 +1,17 @@
 mandoc: name.in:11:5: ERROR: escaped character not allowed in a name: bs\e
-mandoc: name.in:13:5: ERROR: escaped character not allowed in a name: bl\ 
-mandoc: name.in:16:29: WARNING: invalid escape sequence: \*[norm
-mandoc: name.in:16:28: STYLE: whitespace at end of input line
-mandoc: name.in:18:7: WARNING: undefined string, using "": quot
-mandoc: name.in:18:6: STYLE: whitespace at end of input line
-mandoc: name.in:26:6: WARNING: undefined string, using "": bse
-mandoc: name.in:26:5: STYLE: whitespace at end of input line
-mandoc: name.in:28:5: WARNING: undefined string, using "": bs
-mandoc: name.in:28:4: STYLE: whitespace at end of input line
-mandoc: name.in:32:7: WARNING: undefined string, using "": bl e
-mandoc: name.in:32:6: STYLE: whitespace at end of input line
-mandoc: name.in:34:6: WARNING: undefined string, using "": ble
-mandoc: name.in:34:5: STYLE: whitespace at end of input line
-mandoc: name.in:36:5: WARNING: undefined string, using "": bl
-mandoc: name.in:36:4: STYLE: whitespace at end of input line
+mandoc: name.in:14:5: ERROR: escaped character not allowed in a name: bl\ 
+mandoc: name.in:18:29: WARNING: invalid escape sequence: \*[norm
+mandoc: name.in:18:28: STYLE: whitespace at end of input line
+mandoc: name.in:20:7: WARNING: undefined string, using "": quot
+mandoc: name.in:20:6: STYLE: whitespace at end of input line
+mandoc: name.in:28:6: WARNING: undefined string, using "": bse
+mandoc: name.in:28:5: STYLE: whitespace at end of input line
+mandoc: name.in:30:5: WARNING: undefined string, using "": bs
+mandoc: name.in:30:4: STYLE: whitespace at end of input line
+mandoc: name.in:38:7: WARNING: undefined string, using "": bl e
+mandoc: name.in:38:6: STYLE: whitespace at end of input line
+mandoc: name.in:40:6: WARNING: undefined string, using "": ble
+mandoc: name.in:40:5: STYLE: whitespace at end of input line
+mandoc: name.in:42:5: WARNING: undefined string, using "": bl
+mandoc: name.in:42:4: STYLE: whitespace at end of input line
+mandoc: name.in:46:19: WARNING: undefined escape, printing literally: \G