Markdown.pl: do not recognize '_' within words

The '*' character can still be used for emphasis within
a word, but '_' will remain unchanged unless it starts
and ends a word.

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

Markdown.pl

@@ -1048,11 +1048,15 @@ sub _DoItalicsAndBold {
 	my $text = shift;
 
 	# <strong> must go first:
-	$text =~ s{ (\*\*|__) (?=\S) (.+?[*_]*) (?<=\S) \1 }
-		{<strong>$2</strong>}gsx;
-
-	$text =~ s{ (\*|_) (?=\S) (.+?) (?<=\S) \1 }
-		{<em>$2</em>}gsx;
+	$text =~ s{ \*\* (?=\S) (.+?[*_]*) (?<=\S) \*\* }
+		{<strong>$1</strong>}gsx;
+	$text =~ s{ (?<!\w) __ (?=\S) (.+?[*_]*) (?<=\S) __ (?!\w) }
+		{<strong>$1</strong>}gsx;
+
+	$text =~ s{ \* (?=\S) (.+?) (?<=\S) \* }
+		{<em>$1</em>}gsx;
+	$text =~ s{ (?<!\w) _ (?=\S) (.+?) (?<=\S) _ (?!\w) }
+		{<em>$1</em>}gsx;
 
 	return $text;
 }

syntax.text

@@ -740,8 +740,9 @@ will produce:
 
 You can use whichever style you prefer; the lone restriction is that
 the same character must be used to open and close an emphasis span.
+Additionally `_` and double `_` are not recognized within words.
 
-Emphasis can be used in the middle of a word:
+Emphasis using `*` can be used in the middle of a word:
 
     un*frigging*believable