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, 43 insertions, 0 deletions

diff --git a/regress/roff/esc/invalid.out_lint b/regress/roff/esc/invalid.out_lint
new file mode 100644
index 00000000..a0afa7ae
--- /dev/null
+++ b/regress/roff/esc/invalid.out_lint
@@ -0,0 +1,43 @@
+mandoc: invalid.in:7:11: WARNING: invalid escape sequence: \[+]
+mandoc: invalid.in:7:8: WARNING: undefined escape, printing literally: \+
+mandoc: invalid.in:8:16: WARNING: invalid escape sequence: \[;]
+mandoc: invalid.in:8:13: WARNING: undefined escape, printing literally: \;
+mandoc: invalid.in:9:16: WARNING: invalid escape sequence: \[<]
+mandoc: invalid.in:9:13: WARNING: undefined escape, printing literally: \<
+mandoc: invalid.in:10:15: WARNING: invalid escape sequence: \[=]
+mandoc: invalid.in:10:12: WARNING: undefined escape, printing literally: \=
+mandoc: invalid.in:11:19: WARNING: invalid escape sequence: \[>]
+mandoc: invalid.in:11:16: WARNING: undefined escape, printing literally: \>
+mandoc: invalid.in:12:9: WARNING: invalid escape sequence: \[@]
+mandoc: invalid.in:12:6: WARNING: undefined escape, printing literally: \@
+mandoc: invalid.in:13:18: WARNING: undefined escape, printing literally: \]
+mandoc: invalid.in:14:21: WARNING: invalid escape sequence: \[}]
+mandoc: invalid.in:14:16: WARNING: invalid escape sequence: \[{]
+mandoc: invalid.in:15:12: WARNING: invalid escape sequence: \[1]
+mandoc: invalid.in:15:9: WARNING: undefined escape, printing literally: \1
+mandoc: invalid.in:16:8: WARNING: invalid escape sequence: \[G]
+mandoc: invalid.in:16:5: WARNING: undefined escape, printing literally: \G
+mandoc: invalid.in:17:8: WARNING: invalid escape sequence: \[I]
+mandoc: invalid.in:17:5: WARNING: undefined escape, printing literally: \I
+mandoc: invalid.in:18:8: WARNING: invalid escape sequence: \[i]
+mandoc: invalid.in:18:5: WARNING: undefined escape, printing literally: \i
+mandoc: invalid.in:19:8: WARNING: invalid escape sequence: \[J]
+mandoc: invalid.in:19:5: WARNING: undefined escape, printing literally: \J
+mandoc: invalid.in:20:8: WARNING: invalid escape sequence: \[j]
+mandoc: invalid.in:20:5: WARNING: undefined escape, printing literally: \j
+mandoc: invalid.in:21:8: WARNING: invalid escape sequence: \[K]
+mandoc: invalid.in:21:5: WARNING: undefined escape, printing literally: \K
+mandoc: invalid.in:22:8: WARNING: invalid escape sequence: \[P]
+mandoc: invalid.in:22:5: WARNING: undefined escape, printing literally: \P
+mandoc: invalid.in:23:8: WARNING: invalid escape sequence: \[Q]
+mandoc: invalid.in:23:5: WARNING: undefined escape, printing literally: \Q
+mandoc: invalid.in:24:8: WARNING: invalid escape sequence: \[q]
+mandoc: invalid.in:24:5: WARNING: undefined escape, printing literally: \q
+mandoc: invalid.in:25:8: WARNING: invalid escape sequence: \[T]
+mandoc: invalid.in:25:5: WARNING: undefined escape, printing literally: \T
+mandoc: invalid.in:26:8: WARNING: invalid escape sequence: \[U]
+mandoc: invalid.in:26:5: WARNING: undefined escape, printing literally: \U
+mandoc: invalid.in:27:8: WARNING: invalid escape sequence: \[W]
+mandoc: invalid.in:27:5: WARNING: undefined escape, printing literally: \W
+mandoc: invalid.in:28:8: WARNING: invalid escape sequence: \[y]
+mandoc: invalid.in:28:5: WARNING: undefined escape, printing literally: \y