support dot within plain (unquoted) keys (#430)

* support dot within plain (unquoted) keys

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ms.author: author
ms.date: yyyy/MM/dd
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* double slash

Co-authored-by: javaor <69024684+javaor@users.noreply.github.com>

flexmark-ext-yaml-front-matter/src/main/java/com/vladsch/flexmark/ext/yaml/front/matter/internal/YamlFrontMatterBlockParser.java

@@ -21,7 +21,7 @@
 import java.util.regex.Pattern;
 
 public class YamlFrontMatterBlockParser extends AbstractBlockParser {
-    final private static Pattern REGEX_METADATA = Pattern.compile("^[ ]{0,3}([A-Za-z0-9_-]+):\\s*(.*)");
+    final private static Pattern REGEX_METADATA = Pattern.compile("^[ ]{0,3}([A-Za-z0-9_\\-.]+):\\s*(.*)");
     final private static Pattern REGEX_METADATA_LIST = Pattern.compile("^[ ]+-\\s*(.*)");
     final private static Pattern REGEX_METADATA_LITERAL = Pattern.compile("^\\s*(.*)");
     final private static Pattern REGEX_BEGIN = Pattern.compile("^-{3}(\\s.*)?");