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, 15 insertions, 0 deletions

diff --git a/regress/roff/esc/O.in b/regress/roff/esc/O.in
new file mode 100644
index 00000000..d446ebb6
--- /dev/null
+++ b/regress/roff/esc/O.in
@@ -0,0 +1,15 @@
+.\" $OpenBSD$
+.TH ESC-O 1 "December 15, 2018"
+.SH NAME
+esc-O \- escape sequence to suppress output
+.SH DESCRIPTION
+.nf
+O1: a\O1b
+O2: a\O2b
+O3: a\O3b
+O4: a\O4b
+O5: a\O5b
+O52: a\O(52b
+O5n: a\O[5dummy]b
+O6: a\O6b
+O0: a\O0\&\O1b