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, 21 insertions, 0 deletions

diff --git a/regress/char/space/invalid.out_ascii b/regress/char/space/invalid.out_ascii
new file mode 100644
index 00000000..bf3b5a16
--- /dev/null
+++ b/regress/char/space/invalid.out_ascii
@@ -0,0 +1,21 @@
+SPACE-INVALID(1)            General Commands Manual           SPACE-INVALID(1)
+
+
+
+NNAAMMEE
+       SPACE-INVALID - invalid whitespace escape sequences
+
+DDEESSCCRRIIPPTTIIOONN
+       blank: a-bhy]c
+       percent: abc
+       ampersand: abc
+       colon: abc
+       caret: abc
+       underline: a_bc
+       pipe: abc
+       tilde: a bc
+       digit-width: a bc
+
+
+
+OpenBSD                        December 15, 2018              SPACE-INVALID(1)