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, 70 insertions, 8 deletions

diff --git a/regress/roff/cond/register.in b/regress/roff/cond/register.in
index 879c573c..7f137c9c 100644
--- a/regress/roff/cond/register.in
+++ b/regress/roff/cond/register.in
@@ -1,5 +1,5 @@
-.\" $OpenBSD: register.in,v 1.3 2019/02/06 20:54:28 schwarze Exp $
-.TH REGISTER 1 "February 6, 2019"
+.\" $OpenBSD: register.in,v 1.4 2022/06/03 11:50:25 schwarze Exp $
+.TH REGISTER 1 "June 3, 2022"
 .SH NAME
 register \- conditional testing whether a register is defined
 .SH DESCRIPTION
@@ -11,10 +11,35 @@ register \- conditional testing whether a register is defined
 .el OOPS
 .if !rmyreg OOPS
 .PP
-identifier + identifier:
+tab after identifier:
 .ie rmyreg	myreg is defined
 .el OOPS
 .PP
 escape sequence after identifier:
 .ie rmyreg\(enmyreg is defined
 .el OOPS
+.PP
+backslash in name:
+.nr \\ 0
+.ie r\\ \e is defined
+.el OOPS
+.rr \\
+.if r\\ is still defined!?
+.PP
+dot in name:
+.nr . 0
+.ie r. \&. is defined
+.el OOPS
+.ie r\. \e. is defined
+.el OOPS
+.rr \.
+.if r. is still defined!?
+.PP
+invalid escape in name:
+.nr G 0
+.ie rG G is defined
+.el OOPS
+.ie r\G \eG is defined
+.el OOPS
+.rr \G
+.if rG is still defined!?
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)
diff --git a/regress/roff/cond/string.in b/regress/roff/cond/string.in
index 273984be..fedb4d81 100644
--- a/regress/roff/cond/string.in
+++ b/regress/roff/cond/string.in
@@ -1,5 +1,5 @@
-.\" $OpenBSD: string.in,v 1.4 2019/02/06 20:54:28 schwarze Exp $
-.TH STRING 1 "February 6, 2019"
+.\" $OpenBSD: string.in,v 1.5 2022/06/03 11:50:25 schwarze Exp $
+.TH STRING 1 "June 3, 2022"
 .SH NAME
 string \- conditional testing whether a string is defined
 .SH DESCRIPTION
@@ -40,3 +40,28 @@ identifier and tab:
 escape sequence after identifier:
 .ie d mystr\(enmystr is defined
 .el OOPS
+.PP
+backslash in name:
+.ds \\ value
+.ie d \\ \e is defined
+.el OOPS
+.rm \\
+.if d \\ still defined!?
+.PP
+dot in name:
+.ds . value
+.ie d . \&. is defined
+.el OOPS
+.ie d \. \e. is defined
+.el OOPS
+.rm .
+.if d . still defined!?
+.PP
+invalid escape in name:
+.ds G value
+.ie d G G is defined
+.el OOPS
+.ie d \G \eG is defined
+.el OOPS
+.rm \G
+.if d G still defined!?
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)