send-email: align RFC 2047 decoding more closely with the spec

More specifically:

* Add "\" to the list of characters not allowed in a token (see RFC 2047
  errata).

* Share regexes between unquote_rfc2047 and is_rfc2047_quoted. Besides
  removing duplication, this also makes unquote_rfc2047 more stringent.

* Allow both "q" and "Q" to identify the encoding.

* Allow lowercase hexadecimal digits in the "Q" encoding.

And, more on the cosmetic side:

* Change the "encoded-text" regex to exclude rather than include characters,
  for clarity and consistency with "token".

Signed-off-by: Роман Донченко <dpb@corrigendum.ru>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -143,6 +143,11 @@ my $have_mail_address = eval { require Mail::Address; 1 };
 my $smtp;
 my $auth;
 
+# Regexes for RFC 2047 productions.
+my $re_token = qr/[^][()<>@,;:\\"\/?.= \000-\037\177-\377]+/;
+my $re_encoded_text = qr/[^? \000-\037\177-\377]+/;
+my $re_encoded_word = qr/=\?($re_token)\?($re_token)\?($re_encoded_text)\?=/;
+
 # Variables we fill in automatically, or via prompting:
 my (@to,$no_to,@initial_to,@cc,$no_cc,@initial_cc,@bcclist,$no_bcc,@xh,
 	$initial_reply_to,$initial_subject,@files,
@@ -906,15 +911,20 @@ $time = time - scalar $#files;
 
 sub unquote_rfc2047 {
 	local ($_) = @_;
-	my $encoding;
-	s{=\?([^?]+)\?q\?(.*?)\?=}{
-		$encoding = $1;
-		my $e = $2;
-		$e =~ s/_/ /g;
-		$e =~ s/=([0-9A-F]{2})/chr(hex($1))/eg;
-		$e;
+	my $charset;
+	s{$re_encoded_word}{
+		$charset = $1;
+		my $encoding = $2;
+		my $text = $3;
+		if ($encoding eq 'q' || $encoding eq 'Q') {
+			$text =~ s/_/ /g;
+			$text =~ s/=([0-9A-F]{2})/chr(hex($1))/egi;
+			$text;
+		} else {
+			$&; # other encodings not supported yet
+		}
 	}eg;
-	return wantarray ? ($_, $encoding) : $_;
+	return wantarray ? ($_, $charset) : $_;
 }
 
 sub quote_rfc2047 {
@@ -927,10 +937,8 @@ sub quote_rfc2047 {
 
 sub is_rfc2047_quoted {
 	my $s = shift;
-	my $token = qr/[^][()<>@,;:"\/?.= \000-\037\177-\377]+/;
-	my $encoded_text = qr/[!->@-~]+/;
 	length($s) <= 75 &&
-	$s =~ m/^(?:"[[:ascii:]]*"|=\?$token\?$token\?$encoded_text\?=)$/o;
+	$s =~ m/^(?:"[[:ascii:]]*"|$re_encoded_word)$/o;
 }
 
 sub subject_needs_rfc2047_quoting {