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, 0 deletions

diff --git a/mdoc_markdown.c b/mdoc_markdown.c
index b5a856a1..07a5f907 100644
--- a/mdoc_markdown.c
+++ b/mdoc_markdown.c
@@ -589,6 +589,9 @@ md_word(const char *s)
 			case ESCAPE_SPECIAL:
 				uc = mchars_spec2cp(seq, sz);
 				break;
+			case ESCAPE_UNDEF:
+				uc = *seq;
+				break;
 			case ESCAPE_DEVICE:
 				md_rawword("markdown");
 				continue;