Allow underscores in entity names, and entities with no relationships

src/diagrams/er/parser/erDiagram.jison

@@ -27,7 +27,7 @@ o\{                       return 'ZERO_OR_MORE';
 \-\-                      return 'IDENTIFYING';
 \.\-                      return 'NON_IDENTIFYING';
 \-\.                      return 'NON_IDENTIFYING';
-[A-Za-z][A-Za-z0-9\-]*    return 'ALPHANUM';
+[A-Za-z][A-Za-z0-9\-_]*    return 'ALPHANUM';
 .                         return yytext[0];
 <<EOF>>                   return 'EOF';
 
@@ -67,6 +67,7 @@ statement
           yy.addRelationship($1, $5, $3, $2);
           /*console.log($1 + $2 + $3 + ':' + $5);*/
       }
+    | entityName { yy.addEntity($1); }
     ;
 
 entityName

src/diagrams/er/parser/erDiagram.spec.js

@@ -14,6 +14,25 @@ describe('when parsing ER diagram it...', function() {
     erDiagram.parser.yy.clear();
   });
 
+  it ('should allow stand-alone entities with no relationships', function() {
+    const line1 = 'ISLAND';
+    const line2 = 'MAINLAND';
+    erDiagram.parser.parse(`erDiagram\n${line1}\n${line2}`);
+
+    expect(Object.keys(erDb.getEntities()).length).toBe(2);
+    expect (erDb.getRelationships().length).toBe(0);
+  });
+
+  it ('should allow hyphens and underscores in entity names', function() {
+    const line1 = 'DUCK-BILLED-PLATYPUS';
+    const line2 = 'CHARACTER_SET';
+    erDiagram.parser.parse(`erDiagram\n${line1}\n${line2}`);
+
+    const entities = erDb.getEntities();
+    expect (entities["DUCK-BILLED-PLATYPUS"]).toBe('DUCK-BILLED-PLATYPUS');
+    expect (entities.CHARACTER_SET).toBe('CHARACTER_SET');
+  });
+
   it('should associate two entities correctly', function() {
     erDiagram.parser.parse('erDiagram\nCAR ||--o{ DRIVER : "insured for"');
     const entities = erDb.getEntities();