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, 28 insertions, 0 deletions

diff --git a/regress/roff/esc/invalid.in b/regress/roff/esc/invalid.in
new file mode 100644
index 00000000..b2e77159
--- /dev/null
+++ b/regress/roff/esc/invalid.in
@@ -0,0 +1,28 @@
+.\" $OpenBSD$
+.TH ESC-INVALID 1 "December 15, 2018"
+.SH NAME
+esc-invalid \- invalid roff escape sequences
+.SH DESCRIPTION
+.nf
+plus: a\+b\[+]c
+semicolon: a\;b\[;]c
+less than: a\<b\[<]c
+equal to: a\=b\[=]c
+greater than: a\>b\[>]c
+at: a\@b\[@]c
+square bracket: a\]b
+curly braces: a\[{]b\[}]c
+digit: a\1b\[1]c
+G: a\Gb\[G]c
+I: a\Ib\[I]c
+i: a\ib\[i]c
+J: a\Jb\[J]c
+j: a\jb\[j]c
+K: a\Kb\[K]c
+P: a\Pb\[P]c
+Q: a\Qb\[Q]c
+q: a\qb\[q]c
+T: a\Tb\[T]c
+U: a\Ub\[U]c
+W: a\Wb\[W]c
+y: a\yb\[y]c