diff --git a/src/Property/Selector.php b/src/Property/Selector.php
index 44355e63..295a7f53 100644
--- a/src/Property/Selector.php
+++ b/src/Property/Selector.php
@@ -62,6 +62,7 @@ class Selector
                 [a-zA-Z0-9\x{00A0}-\x{FFFF}_^$|*="\'~\[\]()\-\s\.:#+>]* # any sequence of valid unescaped characters
                 (?:\\\\.)?                                              # a single escaped character
                 (?:([\'"]).*?(?<!\\\\)\2)?                              # a quoted text like [id="example"]
+                (?:\(.*?\))?                                            # an argument for pseudo selector like :not(a,b)
             )*
         )$
         /ux';
diff --git a/src/RuleSet/DeclarationBlock.php b/src/RuleSet/DeclarationBlock.php
index 8a1da14e..523337b8 100644
--- a/src/RuleSet/DeclarationBlock.php
+++ b/src/RuleSet/DeclarationBlock.php
@@ -98,7 +98,19 @@ public function setSelectors($mSelector, $oList = null)
         if (is_array($mSelector)) {
             $this->aSelectors = $mSelector;
         } else {
-            $this->aSelectors = explode(',', $mSelector);
+            list($sSelectors, $aPlaceholders) = $this->addSelectorExpressionPlaceholders($mSelector);
+            if (empty($aPlaceholders)) {
+                $this->aSelectors = explode(',', $sSelectors);
+            } else {
+                $aSearches = array_keys($aPlaceholders);
+                $aReplaces = array_values($aPlaceholders);
+                $this->aSelectors = array_map(
+                    static function ($sSelector) use ($aSearches, $aReplaces) {
+                        return str_replace($aSearches, $aReplaces, $sSelector);
+                    },
+                    explode(',', $sSelectors)
+                );
+            }
         }
         foreach ($this->aSelectors as $iKey => $mSelector) {
             if (!($mSelector instanceof Selector)) {
@@ -125,6 +137,56 @@ public function setSelectors($mSelector, $oList = null)
         }
     }
 
+    /**
+     * Add placeholders for parenthetical expressions in selectors which may contain commas that break exploding.
+     *
+     * This prevents a single selector like `.widget:not(.foo, .bar)` from erroneously getting parsed in setSelectors as
+     * two selectors `.widget:not(.foo` and `.bar)`.
+     *
+     * @param string $sSelectors Selectors.
+     * @return array First array value is the selectors with placeholders, and second value is the array of placeholders
+     *               mapped to the original expressions.
+     */
+    private function addSelectorExpressionPlaceholders($sSelectors)
+    {
+        $iOffset = 0;
+        $aPlaceholders = [];
+
+        while (preg_match('/\(|\[/', $sSelectors, $aMatches, PREG_OFFSET_CAPTURE, $iOffset)) {
+            $sMatchString = $aMatches[0][0];
+            $iMatchOffset = $aMatches[0][1];
+            $iStyleLength = strlen($sSelectors);
+            $iOpenParens  = 1;
+            $iStartOffset = $iMatchOffset + strlen($sMatchString);
+            $iFinalOffset = $iStartOffset;
+            for (; $iFinalOffset < $iStyleLength; $iFinalOffset++) {
+                if ('(' === $sSelectors[ $iFinalOffset ] || '[' === $sSelectors[ $iFinalOffset ]) {
+                    $iOpenParens++;
+                } elseif (')' === $sSelectors[ $iFinalOffset ] || ']' === $sSelectors[ $iFinalOffset ]) {
+                    $iOpenParens--;
+                }
+
+                // Found the end of the expression, so replace it with a placeholder.
+                if (0 === $iOpenParens) {
+                    $sMatchedExpr = substr($sSelectors, $iMatchOffset, $iFinalOffset - $iMatchOffset + 1);
+                    $sPlaceholder = sprintf('{placeholder:%d}', count($aPlaceholders) + 1);
+                    $aPlaceholders[ $sPlaceholder ] = $sMatchedExpr;
+
+                    // Update the CSS to replace the matched calc() with the placeholder function.
+                    $sSelectors = substr($sSelectors, 0, $iMatchOffset)
+                                . $sPlaceholder
+                                . substr($sSelectors, $iFinalOffset + 1);
+                    // Update offset based on difference of length of placeholder vs original matched calc().
+                    $iFinalOffset += strlen($sPlaceholder) - strlen($sMatchedExpr);
+                    break;
+                }
+            }
+            // Start matching at the next byte after the match.
+            $iOffset = $iFinalOffset + 1;
+        }
+        return [ $sSelectors, $aPlaceholders ];
+    }
+
     /**
      * Remove one of the selectors of the block.
      *
diff --git a/tests/ParserTest.php b/tests/ParserTest.php
index c8e8ac76..f3c5842f 100644
--- a/tests/ParserTest.php
+++ b/tests/ParserTest.php
@@ -256,6 +256,12 @@ public function specificity()
                 case "li.green":
                     self::assertSame(11, $oSelector->getSpecificity());
                     break;
+                case "div:not(.foo[title=\"a,b\"], .bar)":
+                    self::assertSame(31, $oSelector->getSpecificity());
+                    break;
+                case "div[title=\"a,b\"]":
+                    self::assertSame(11, $oSelector->getSpecificity());
+                    break;
                 default:
                     self::fail("specificity: untested selector " . $oSelector->getSelector());
             }
@@ -272,13 +278,20 @@ public function specificity()
             new Selector('.help:hover', true),
             new Selector('li.green', true),
             new Selector('ol li::before', true),
+            new Selector('div:not(.foo[title="a,b"], .bar)', true),
+            new Selector('div[title="a,b"]', true),
         ], $oDoc->getSelectorsBySpecificity('<= 100'));
         self::assertEquals([
             new Selector('.help:hover', true),
             new Selector('li.green', true),
             new Selector('ol li::before', true),
+            new Selector('div:not(.foo[title="a,b"], .bar)', true),
+            new Selector('div[title="a,b"]', true),
         ], $oDoc->getSelectorsBySpecificity('< 100'));
-        self::assertEquals([new Selector('li.green', true)], $oDoc->getSelectorsBySpecificity('11'));
+        self::assertEquals([
+            new Selector('li.green', true),
+            new Selector('div[title="a,b"]', true),
+        ], $oDoc->getSelectorsBySpecificity('11'));
         self::assertEquals([new Selector('ol li::before', true)], $oDoc->getSelectorsBySpecificity('3'));
     }
 
diff --git a/tests/fixtures/specificity.css b/tests/fixtures/specificity.css
index 82a2939a..df03ff0f 100644
--- a/tests/fixtures/specificity.css
+++ b/tests/fixtures/specificity.css
@@ -2,6 +2,8 @@
 #file,
 .help:hover,
 li.green,
-ol li::before {
+ol li::before,
+div:not(.foo[title="a,b"], .bar),
+div[title="a,b"] {
 	font-family: Helvetica;
 }