diff --git a/lib/flipcss.js b/lib/flipcss.js
index 69233f0..64efe23 100644
--- a/lib/flipcss.js
+++ b/lib/flipcss.js
@@ -13,32 +13,57 @@ var flipcss = {
      * (?!y) -> not followed by y.
      * @private
      */
-    _ruleMatchIgnorePattern: "(?![^\n]*/\\*\\s*!"
-                           + "(direction-ignore|rtl-only|ltr-only)"
-                           + ")",
+    _ruleMatchIgnorePattern: "(?![^\n]*/\\*\\s*!" +
+            "(direction-ignore|rtl-only|ltr-only)" +
+            ")",
 
 
     /**
-     * Swap two words in a string
+     * Swap two words in a string. Used to swap "left" and "right", and
+     * vice versa. This function is pretty naive, and just swaps every instance
+     * of the words that it comes across, with some exceptions:
+     * - Words where word1 or word2 are parts of other words (like "copyright")
+     *   are left alone. Words like "margin-left" are not.
+     * - You can set some flags to leave the words to be swapped unchanged
+     *   in some contexts.
      *
      * @private
      * @param {String} string String to process
-     * @param {String} word1 Word to swap with word2
-     * @param {String} word2 Word to swap with word1
+     * @param {String} word1 Word to swap with word2 (alphanumeric chars only)
+     * @param {String} word2 Word to swap with word1 (alphanumeric chars only)
+     * @param {Boolean} flipFilenames
      * @returns {String} Processed string.
-     *
-     * NOTE: This function is stupid; word1 and word2 are expected to be
-     * alphanumeric characters, but no checking is performed, so it may
-     * break on other strings.
      */
-    _swapWords: function(string, word1, word2) {
-        // Matches one of the given words, nothing else (i.e. the rest of the
-        // rule is ignored), except ignore pattern.
-        // Captures the found word.
-        var pattern = new RegExp(
-              "([^a-z0-9])(" + word1 + "|" + word2 + ")([^a-z0-9])"
-            + this._ruleMatchIgnorePattern, "g");
+    _swapWords: function(string, word1, word2, flipFilenames, flipSelectors) {
+        // Regexp parts
+        var rp = [];
+
+        // Should only match whole words, or words separated by e.g. a hyphen
+        // ("margin-left" would be matched, but not "copyright"). Thus, we
+        // need to check for alphanumeric chars right before and right
+        // after the word:
+        var noAlphaNum = "([^a-z0-9])";
+        rp.push(noAlphaNum); // Charactes not allowed right before word
+
+        // The two words to swap
+        rp.push("(" + word1 + "|" + word2 + ")");
+
+        // If filenames are not to be flipped:
+        if (!flipFilenames) {
+            rp.push("(?!.*\\))"); // Not inside parantheses (e.g. "url()")
+        }
 
+/*        if (!flipSelectors) {
+            rp.push("(?!(.*\\s){)"); // Not inside parantheses (e.g. "url()")
+        }*/
+
+        rp.push(noAlphaNum); // Alphanumerics not allowed right before word
+
+        // Ignore pattern, in case this specific rule is to be ignored.
+        rp.push(this._ruleMatchIgnorePattern);
+
+        // Do replace
+        var pattern = new RegExp(rp.join(""), "g");
         return string.replace(pattern, function(_, pre, word, post) {
             return pre + (word === word1 ? word2 : word1) + post;
         });
@@ -272,13 +297,16 @@ module.exports = {
      * @param {Boolean} flipPseudo Flip :before and :after.
      * @returns {String} Flipped CSS.
      */
-    flip: function(string, warnings, flipPseudo) {
+    flip: function(string, warnings, flipPseudo, flipFilenames) {
         if (!warnings) {
             warnings = false;
         }
         if (!flipPseudo) {
             flipPseudo = false;
         }
+        if (!flipFilenames) {
+            flipFilenames = false;
+        }
 
         // Output warnings:
         if (warnings) {
@@ -294,7 +322,7 @@ module.exports = {
         string = flipcss._cleanLineFeeds(string);
 
         // Do processing
-        string = flipcss._swapWords(string, "left", "right");
+        string = flipcss._swapWords(string, "left", "right", flipFilenames);
         string = flipcss._swapValues(string);
         string = flipcss._swapBackgroundPosition(string);
         string = flipcss._swapBeforeAfter(string, flipPseudo);
diff --git a/test/test.js b/test/test.js
index 799f48b..2658adf 100644
--- a/test/test.js
+++ b/test/test.js
@@ -33,6 +33,17 @@ buster.assertions.add("flipsTo", {
 });
 
 
+buster.assertions.add("flipsFilename", {
+    assert: function (input, expectedOutput) {
+        this.output = lib.flip(input, false, false, true);
+        return this.output === expectedOutput;
+    },
+    assertMessage: "Expected \"${0}\" to flip to \"${1}\", got \"${output}\".",
+    refuteMessage: "Expected \"${0}\" to not flip to \"${1}\","
+        + " got \"${output}\"."
+});
+
+
 buster.assertions.add("flipsPseudo", {
     assert: function (input, expectedOutput) {
         this.output = lib.flip(input, false, true);
@@ -120,15 +131,22 @@ buster.testCase("CSS word swapper", {
         assert.flipsTo(".copyright { float: right; }",
                        ".copyright { float: left; }");
         // "rights.png" Should not be changed (subword)
-        assert.flipsTo("background: url('rights.png')",
-                       "background: url('rights.png')");
-        // "arrow-left.png" Should be changed (subword)
-        assert.flipsTo("background: url('arrow-left.png')",
-                       "background: url('arrow-right.png')");
+        assert.flipsTo("/* rights */')",
+                       "/* rights */')");
         // "pull-right" should be changed (subword), and float should be changed
         assert.flipsTo(".pull-right { float: right; }",
                        ".pull-left { float: left; }");
     },
+    "does not swap filenames by default": function() {
+        assert.flipsTo("background: url('arrow-left.png')",
+                       "background: url('arrow-left.png')");
+    },
+    "swaps filenames when asked to": function() {
+        assert.flipsFilename("background: url('arrow-left.png')",
+                       "background: url('arrow-right.png')");
+        assert.flipsFilename("background: url('left-imgs/arrow.png')",
+                       "background: url('right-imgs/arrow.png')");
+    },
     "leaves ignored rules alone": function() {
         // Basic case: Nothing should change.
         assert.flipsTo(".foo { clear: left; /* !direction-ignore */ }",