Markdown.pl: eliminate egregious URL snafus

The standard explicitly prohibits nesting of "a" tags.

Prevent nesting from occuring when only "markdown" input
is present.  If explicit "<a ...>...</a>" tags are present
in the input they will be (mostly) left alone even if
they've been incorrectly nested.

Avoid producing mojibake links in the case that the URL
itself appears to contain another URL.  (The wayback
machine links often look like this.)

In essence, once a link (either an "a" or "img") tag has been
generated/processed, avoid processing it again in order to
make sure no accidental mojibake occurs.

One consequence of this change is that "Automatic Links"
that are NOT surrounded by '<' and '>' will now only be
recognized if they occur at the beginning of the input or
after a whitespace (a newline qualifies) character.

This also helps to eliminate unintended double linkification.

Furthermore, URLs containing peculiar characters in them (e.g.
single quote and/or double quote) should be far less
troublesome now as well.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

Markdown.pl

@@ -115,7 +115,7 @@ BEGIN {
 my %g_escape_table;
 BEGIN {
     $g_escape_table{""} = "\2\3";
-    foreach my $char (split //, "\\\`*_~{}[]()>#+-.!|:") {
+    foreach my $char (split //, "\\\`*_~{}[]()>#+-.!|:<") {
 	$g_escape_table{$char} = block_id($char,1);
     }
 }
@@ -568,10 +568,10 @@ sub _StripLinkDefinitions {
 	my $title = _strip($5);
 	$url =~ s/\\\n\s*//gs;
 	if ($id ne "") {
-		$g_urls{$id} = _EncodeAmpsAndAngles($url);
+		# These values always get passed through _MakeATag or _MakeIMGTag later
+		$g_urls{$id} = $url;
 		if (defined($title) && $title ne "") {
 		    $g_titles{$id} = $title;
-		    $g_titles{$id} =~ s/\042/&quot;/g;
 		}
 	}
     }
@@ -818,10 +818,12 @@ sub _MakeATag {
 	defined($text) or $text="";
 	defined($title) or $title="";
 
-	my $result = "<a href=\"" . _EncodeAttText($url) . "\"";
+	my $result = $g_escape_table{'<'}."a href=\"" . _EncodeAttText($url) . "\"";
 	$title = _strip($title);
+	$text =~ s{<(/?a)}{&lt;$1}sogi;
 	$result .= " title=\"" . _EncodeAttText($title) . "\"" if $title ne "";
-	return $result . ">" . $text . "</a>";
+	return $result . $g_escape_table{'>'} .
+	    $text . $g_escape_table{'<'}."/a".$g_escape_table{'>'};
 }
 
 
@@ -977,7 +979,7 @@ sub _MakeIMGTag {
     defined($title) or $title="";
     return "" unless $url ne "";
 
-    my $result = "<img src=\"" . _EncodeAttText($url) . "\"";
+    my $result = $g_escape_table{'<'}."img src=\"" . _EncodeAttText($url) . "\"";
     my ($w, $h) = (0, 0);
     ($alt, $title) = (_strip($alt), _strip($title));
     if ($title =~ /^(.*)\(([1-9][0-9]*)[xX]([1-9][0-9]*)\)$/os) {
@@ -991,7 +993,8 @@ sub _MakeIMGTag {
     $result .= " width=\"$w\"" if $w != 0;
     $result .= " height=\"$h\"" if $h != 0;
     $result .= " title=\"" . _EncodeAttText($title) . "\"" if $title ne "";
-    $result .= $opt{empty_element_suffix};
+    $result .= " /" unless $opt{empty_element_suffix} eq ">";
+    $result .= $g_escape_table{'>'};
     return $result;
 }
 
@@ -2025,11 +2028,13 @@ sub _ProcessURLTag {
 	    while ($tag =~ /\G([^\s\042\047>]+=)([\042\047])((?:(?!\2)(?!>).)*)(\2\s*)/gc) {
 		    my ($p, $q, $v, $s) = ($1, $2, $3, $4);
 		    if (lc($p) eq $att && $v ne "") {
-			    $v = _HTMLEncode(_PrefixURL($v));
+			    $v = _EncodeAttText(_PrefixURL($v));
 		    }
 		    $out .= $p . $q . $v . $s;
 	    }
 	    $out .= substr($tag, pos($tag));
+	    substr($out,0,1) = $g_escape_table{'<'};
+	    substr($out,-1,1) = $g_escape_table{'>'};
 	    return $out;
     }
 
@@ -2102,7 +2107,7 @@ sub _EncodeBackslashEscapes {
 sub _DoAutoLinks {
     local $_ = shift;
 
-    s{<((https?|ftps?):[^'\042>\s]+)>}{<a href="$1">&lt;$1&gt;</a>}gi;
+    s{<((https?|ftps?):[^'\042>\s]+)>(?!\s*</a>)}{_MakeATag($1, "&lt;".$1."&gt;")}gise;
 
     # Email addresses: <address@domain.foo>
     s{
@@ -2119,10 +2124,10 @@ sub _DoAutoLinks {
     }egix;
 
     # (kjm) I don't do "x" patterns
-    s{(?<![\042'<>])(?<!&[Ll][Tt];)(?<!&#60;)(?<!&#x3[Cc];)\b((?:https?|ftps?)://(?:[-a-zA-Z0-9./?\&\%=_~!*;:\@+\$,\x23](?:(?<![.,:;])|(?=[^\s])))+)}
-     {<a href="$1">$1</a>}sog;
+    s{(?:^|(?<=\s))((?:https?|ftps?)://(?:[-a-zA-Z0-9./?\&\%=_~!*;:\@+\$,\x23](?:(?<![.,:;])|(?=[^\s])))+)}
+     {_MakeATag($1, $1)}soge;
     s{(?<![][])(?<!\] )\[RFC( ?)([0-9]{1,5})\](?![][])(?! \[)}
-     {[<a href="http://tools.ietf.org/html/rfc$2">RFC$1$2</a>]}sog;
+     {"["._MakeATag("https://tools.ietf.org/html/rfc$2", "RFC$1$2", "RFC $2")."]"}soge;
 
     return $_;
 }
@@ -2177,7 +2182,7 @@ sub _EncodeEmailAddress {
 
     # strip the mailto: from the visible part
     (my $bareaddr = $addr) =~ s/^.+?://;
-    $addr = qq{<a href="$addr">$prefix$bareaddr$suffix</a>};
+    $addr = _MakeATag("$addr", $prefix.$bareaddr.$suffix);
 
     return $addr;
 }