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, 6 insertions, 4 deletions

diff --git a/html.c b/html.c
index ce90dfbd..da381a47 100644
--- a/html.c
+++ b/html.c
@@ -1,4 +1,4 @@
-/*	$Id: html.c,v 1.245 2018/12/04 18:29:38 schwarze Exp $ */
+/*	$Id: html.c,v 1.246 2018/12/15 19:30:26 schwarze Exp $ */
 /*
  * Copyright (c) 2008-2011, 2014 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2011-2015, 2017, 2018 Ingo Schwarze <schwarze@openbsd.org>
@@ -402,9 +402,6 @@ print_encode(struct html *h, const char *p, const char *pend, int norecurse)
 			continue;
 
 		esc = mandoc_escape(&p, &seq, &len);
-		if (ESCAPE_ERROR == esc)
-			break;
-
 		switch (esc) {
 		case ESCAPE_FONT:
 		case ESCAPE_FONTPREV:
@@ -422,6 +419,8 @@ print_encode(struct html *h, const char *p, const char *pend, int norecurse)
 		case ESCAPE_SKIPCHAR:
 			h->flags |= HTML_SKIPCHAR;
 			continue;
+		case ESCAPE_ERROR:
+			continue;
 		default:
 			break;
 		}
@@ -446,6 +445,9 @@ print_encode(struct html *h, const char *p, const char *pend, int norecurse)
 			if (c <= 0)
 				continue;
 			break;
+		case ESCAPE_UNDEF:
+			c = *seq;
+			break;
 		case ESCAPE_DEVICE:
 			print_word(h, "html");
 			continue;