[fix refactor] Correctly interpret intent around string interpolation…

… or "meta" objects based on the occurance of the format expressions in the base message to be logged.

lib/winston/logger.js

@@ -14,6 +14,8 @@ var events = require('events'),
     exception = require('./exception'),
     Stream = require('stream').Stream;
 
+const formatRegExp = /%[sdj%]/g;
+
 //
 // ### function Logger (options)
 // #### @options {Object} Options for this instance.
@@ -118,9 +120,9 @@ Logger.prototype.log = function (level) {
   //
   //    logger.info('No interpolation symbols', 'ok', 'why', { meta: 'is-this' });
   //
-  var callback = typeof args[args.length - 1] === 'function' ? args.pop() : null,
-      meta     = typeof args[args.length - 1] === 'object' && Object.prototype.toString.call(args[args.length - 1]) !== '[object RegExp]' ? args.pop() : {},
-      msg      = util.format.apply(null, args);
+  var callback  = typeof args[args.length - 1] === 'function'
+    ? args.pop()
+    : null;
 
   //
   // Handle errors appropriately.
@@ -157,6 +159,21 @@ Logger.prototype.log = function (level) {
     return;
   }
 
+  //
+  // Determining what is `meta` and what are arguments for string interpolation
+  // turns out to be VERY tricky. e.g. in the cases like this:
+  //
+  //    logger.info('No interpolation symbols', 'ok', 'why', { meta: 'is-this' });
+  //
+  var metaType  = Object.prototype.toString.call(args[args.length - 1]),
+      fmtMatch  = args[0].match && args[0].match(formatRegExp),
+      isFormat  = fmtMatch && fmtMatch.length,
+      validMeta = !isFormat
+        ? metaType === '[object Object]' || metaType === '[object Error]' || metaType === '[object Array]'
+        : metaType === '[object Object]',
+      meta = validMeta ? args.pop() : {},
+      msg  = util.format.apply(null, args);
+
   //
   // Respond to the callback.
   //