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, 3 insertions, 3 deletions

diff --git a/regress/char/accent/nocombine.out_utf8 b/regress/char/accent/nocombine.out_utf8
index 3aa441a2..497bf6fd 100644
--- a/regress/char/accent/nocombine.out_utf8
+++ b/regress/char/accent/nocombine.out_utf8
@@ -7,10 +7,10 @@ NNAAMMEE
 
 DDEESSCCRRIIPPTTIIOONN
        bare acute accent: e'e
-       escaped acute accent: e´e
+       escaped acute accent: e´ee
        acute accent sequence: e´e
        bare grave accent: e`e
-       escaped grave accent: e`e
+       escaped grave accent: e`ee
        acute grave sequence: e`e
        hungarian umlaut: e˝e
        circumflex: e^e
@@ -25,4 +25,4 @@ DDEESSCCRRIIPPTTIIOONN
 
 
 
-OpenBSD                          March 8, 2014        CHAR-ACCENT-NOCOMBINE(1)
+OpenBSD                        December 15, 2018      CHAR-ACCENT-NOCOMBINE(1)