Several improvements to escape sequence handling.

* Add the missing special character \_ (underscore).
* Partial implementations of \a (leader character)
and \E (uninterpreted escape character).
* Parse and ignore \r (reverse line feed).
* Add a WARNING message about undefined escape sequences.
* Add an UNSUPP message about unsupported escape sequences.
* Mark \! and \? (transparent throughput)
and \O (suppress output) as unsupported.
* Treat the various variants of zero-width spaces as one-byte escape
sequences rather than as special characters, to avoid defining bogus
forms with square brackets.
* For special characters with one-byte names, do not define bogus
forms with square brackets, except for \[-], which is valid.
* In the form with square brackets, undefined special characters do not
fall back to printing the name verbatim, not even for one-byte names.
* Starting a special character name with a blank is an error.
* Undefined escape sequences never abort formatting of the input
string, not even in HTML output mode.
* Document the newly handled escapes, and a few that were missing.
* Regression tests for most of the above.

1 files changed, 8 insertions, 10 deletions

diff --git a/regress/roff/esc/ignore.in b/regress/roff/esc/ignore.in
index d41be2e1..48507f44 100644
--- a/regress/roff/esc/ignore.in
+++ b/regress/roff/esc/ignore.in
@@ -1,15 +1,13 @@
 .\" $OpenBSD: ignore.in,v 1.3 2017/07/04 14:53:27 schwarze Exp $
-.Dd $Mdocdate: July 4 2017 $
-.Dt ESC-IGNORE 1
-.Os
-.Sh NAME
-.Nm esc-ignore
-.Nd ignored roff escape sequences
-.Sh DESCRIPTION
+.TH ESC-IGNORE 1 "December 15, 2018"
+.SH NAME
+esc-ignore \- ignored roff escape sequences
+.SH DESCRIPTION
+.nf
+closing parenthesis: a\)b\[)]c
+comma: a\,b\[,]c
+slash: a\/b\[/]c
 multiform: a\kxb\k(xyc\k[xyz]d
-.br
 quoted: a\R'myreg 0'b\R'myreg \A'y'0'c
-.br
 sizes: a\s0b\s(12c\s[123]d\s'123'e\s'1\w'xy'2'f
-.br
 signed sizes: a\s-0b\s-(12c\s-[123]d\s-'123'e\s-'1\w'xy'2'f\s-