Explicitly allow class names starting with "--" (#73)

Fixes pugjs/pug#2423.

Author: TimothyGu

index.js

@@ -403,14 +403,14 @@ Lexer.prototype = {
    */
 
   className: function() {
-    var tok = this.scan(/^\.(\-?[_a-z][_a-z0-9\-]*)/i, 'class');
+    var tok = this.scan(/^\.(-?-?[_a-z][_a-z0-9\-]*)/i, 'class');
     if (tok) {
       this.tokens.push(tok);
       this.incrementColumn(tok.val.length);
       return true;
     }
     if (/^\.\-/i.test(this.input)) {
-      this.error('INVALID_CLASS_NAME', 'If a class name begins with a "-", it must be followed by a letter or underscore.');
+      this.error('INVALID_CLASS_NAME', 'If a class name begins with a "-" or "--", it must be followed by a letter or underscore.');
     }
     if (/^\.[0-9]/i.test(this.input)) {
       this.error('INVALID_CLASS_NAME', 'Class names must begin with "-", "_" or a letter.');

test/errors/invalid-class-name-2.json

@@ -1,6 +1,6 @@
 {
-  "msg": "If a class name begins with a \"-\", it must be followed by a letter or underscore.",
+  "msg": "If a class name begins with a \"-\" or \"--\", it must be followed by a letter or underscore.",
   "code": "PUG:INVALID_CLASS_NAME",
   "line": 1,
   "column": 1
-}
+}