diff --git a/wp-includes/pomo/plural-forms.php b/wp-includes/pomo/plural-forms.php
new file mode 100644
index 0000000000..6dbfbfd774
--- /dev/null
+++ b/wp-includes/pomo/plural-forms.php
@@ -0,0 +1,343 @@
+<?php
+
+/**
+ * A gettext Plural-Forms parser.
+ *
+ * @since 4.9.0
+ */
+class Plural_Forms {
+	/**
+	 * Operator characters.
+	 *
+	 * @since 4.9.0
+	 * @var string OP_CHARS Operator characters.
+	 */
+	const OP_CHARS = '|&><!=%?:';
+
+	/**
+	 * Valid number characters.
+	 *
+	 * @since 4.9.0
+	 * @var string NUM_CHARS Valid number characters.
+	 */
+	const NUM_CHARS = '0123456789';
+
+	/**
+	 * Operator precedence.
+	 *
+	 * Operator precedence from highest to lowest. Higher numbers indicate
+	 * higher precedence, and are executed first.
+	 *
+	 * @see https://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B#Operator_precedence
+	 *
+	 * @since 4.9.0
+	 * @var array $op_precedence Operator precedence from highest to lowest.
+	 */
+	protected static $op_precedence = array(
+		'%' => 6,
+
+		'<' => 5,
+		'<=' => 5,
+		'>' => 5,
+		'>=' => 5,
+
+		'==' => 4,
+		'!=' => 4,
+
+		'&&' => 3,
+
+		'||' => 2,
+
+		'?:' => 1,
+		'?' => 1,
+
+		'(' => 0,
+		')' => 0,
+	);
+
+	/**
+	 * Tokens generated from the string.
+	 *
+	 * @since 4.9.0
+	 * @var array $tokens List of tokens.
+	 */
+	protected $tokens = array();
+
+	/**
+	 * Cache for repeated calls to the function.
+	 *
+	 * @since 4.9.0
+	 * @var array $cache Map of $n => $result
+	 */
+	protected $cache = [];
+
+	/**
+	 * Constructor.
+	 *
+	 * @since 4.9.0
+	 *
+	 * @param string $str Plural function (just the bit after `plural=` from Plural-Forms)
+	 */
+	public function __construct( $str ) {
+		$this->parse( $str );
+	}
+
+	/**
+	 * Parse a Plural-Forms string into tokens.
+	 *
+	 * Uses the shunting-yard algorithm to convert the string to Reverse Polish
+	 * Notation tokens.
+	 *
+	 * @since 4.9.0
+	 *
+	 * @param string $str String to parse.
+	 */
+	protected function parse( $str ) {
+		$pos = 0;
+		$len = strlen( $str );
+
+		// Convert infix operators to postfix using the shunting-yard algorithm.
+		$output = array();
+		$stack = array();
+		while ( $pos < $len ) {
+			$next = substr( $str, $pos, 1 );
+
+			switch ( $next ) {
+				// Ignore whitespace
+				case ' ':
+				case "\t":
+					$pos++;
+					break;
+
+				// Variable (n)
+				case 'n':
+					$output[] = [ 'var' ];
+					$pos++;
+					break;
+
+				// Parentheses
+				case '(':
+					$stack[] = $next;
+					$pos++;
+					break;
+
+				case ')':
+					$found = false;
+					while ( ! empty( $stack ) ) {
+						$o2 = $stack[ count( $stack ) - 1 ];
+						if ( $o2 !== '(' ) {
+							$output[] = [ 'op', array_pop( $stack ) ];
+							continue;
+						}
+
+						// Discard open paren.
+						array_pop( $stack );
+						$found = true;
+						break;
+					}
+
+					if ( ! $found ) {
+						throw new Exception( 'Mismatched parentheses' );
+					}
+
+					$pos++;
+					break;
+
+				// Operators
+				case '|':
+				case '&':
+				case '>':
+				case '<':
+				case '!':
+				case '=':
+				case '%':
+				case '?':
+					$end_operator = strspn( $str, self::OP_CHARS, $pos );
+					$operator = substr( $str, $pos, $end_operator );
+					if ( ! array_key_exists( $operator, self::$op_precedence ) ) {
+						throw new Exception( sprintf( 'Unknown operator "%s"', $operator ) );
+					}
+
+					while ( ! empty( $stack ) ) {
+						$o2 = $stack[ count( $stack ) - 1 ];
+
+						// Ternary is right-associative in C
+						if ( $operator === '?:' || $operator === '?' ) {
+							if ( self::$op_precedence[ $operator ] >= self::$op_precedence[ $o2 ] ) {
+								break;
+							}
+						} elseif ( self::$op_precedence[ $operator ] > self::$op_precedence[ $o2 ] ) {
+							break;
+						}
+
+						$output[] = [ 'op', array_pop( $stack ) ];
+					}
+					$stack[] = $operator;
+
+					$pos += $end_operator;
+					break;
+
+				// Ternary "else"
+				case ':':
+					$found = false;
+					$s_pos = count( $stack ) - 1;
+					while ( $s_pos >= 0 ) {
+						$o2 = $stack[ $s_pos ];
+						if ( $o2 !== '?' ) {
+							$output[] = [ 'op', array_pop( $stack ) ];
+							$s_pos--;
+							continue;
+						}
+
+						// Replace.
+						$stack[ $s_pos ] = '?:';
+						$found = true;
+						break;
+					}
+
+					if ( ! $found ) {
+						throw new Exception( 'Missing starting "?" ternary operator' );
+					}
+					$pos++;
+					break;
+
+				// Default - number or invalid
+				default:
+					if ( $next >= '0' && $next <= '9' ) {
+						$span = strspn( $str, self::NUM_CHARS, $pos );
+						$output[] = [ 'value', intval( substr( $str, $pos, $span ) ) ];
+						$pos += $span;
+						continue;
+					}
+
+					throw new Exception( sprintf( 'Unknown symbol "%s"', $next ) );
+			}
+		}
+
+		while ( ! empty( $stack ) ) {
+			$o2 = array_pop( $stack );
+			if ( $o2 === '(' || $o2 === ')' ) {
+				throw new Exception( 'Mismatched parentheses' );
+			}
+
+			$output[] = [ 'op', $o2 ];
+		}
+
+		$this->tokens = $output;
+	}
+
+	/**
+	 * Get the plural form for a number.
+	 *
+	 * Caches the value for repeated calls.
+	 *
+	 * @since 4.9.0
+	 *
+	 * @param int $num Number to get plural form for.
+	 * @return int Plural form value.
+	 */
+	public function get( $num ) {
+		if ( isset( $this->cache[ $num ] ) ) {
+			return $this->cache[ $num ];
+		}
+		return $this->cache[ $num ] = $this->execute( $num );
+	}
+
+	/**
+	 * Execute the plural form function.
+	 *
+	 * @since 4.9.0
+	 *
+	 * @param int $n Variable "n" to substitute.
+	 * @return int Plural form value.
+	 */
+	public function execute( $n ) {
+		$stack = array();
+		$i = 0;
+		$total = count( $this->tokens );
+		while ( $i < $total ) {
+			$next = $this->tokens[$i];
+			$i++;
+			if ( $next[0] === 'var' ) {
+				$stack[] = $n;
+				continue;
+			} elseif ( $next[0] === 'value' ) {
+				$stack[] = $next[1];
+				continue;
+			}
+
+			// Only operators left.
+			switch ( $next[1] ) {
+				case '%':
+					$v2 = array_pop( $stack );
+					$v1 = array_pop( $stack );
+					$stack[] = $v1 % $v2;
+					break;
+
+				case '||':
+					$v2 = array_pop( $stack );
+					$v1 = array_pop( $stack );
+					$stack[] = $v1 || $v2;
+					break;
+
+				case '&&':
+					$v2 = array_pop( $stack );
+					$v1 = array_pop( $stack );
+					$stack[] = $v1 && $v2;
+					break;
+
+				case '<':
+					$v2 = array_pop( $stack );
+					$v1 = array_pop( $stack );
+					$stack[] = $v1 < $v2;
+					break;
+
+				case '<=':
+					$v2 = array_pop( $stack );
+					$v1 = array_pop( $stack );
+					$stack[] = $v1 <= $v2;
+					break;
+
+				case '>':
+					$v2 = array_pop( $stack );
+					$v1 = array_pop( $stack );
+					$stack[] = $v1 > $v2;
+					break;
+
+				case '>=':
+					$v2 = array_pop( $stack );
+					$v1 = array_pop( $stack );
+					$stack[] = $v1 >= $v2;
+					break;
+
+				case '!=':
+					$v2 = array_pop( $stack );
+					$v1 = array_pop( $stack );
+					$stack[] = $v1 != $v2;
+					break;
+
+				case '==':
+					$v2 = array_pop( $stack );
+					$v1 = array_pop( $stack );
+					$stack[] = $v1 == $v2;
+					break;
+
+				case '?:':
+					$v3 = array_pop( $stack );
+					$v2 = array_pop( $stack );
+					$v1 = array_pop( $stack );
+					$stack[] = $v1 ? $v2 : $v3;
+					break;
+
+				default:
+					throw new Exception( sprintf( 'Unknown operator "%s"', $next[1] ) );
+			}
+		}
+
+		if ( count( $stack ) !== 1 ) {
+			throw new Exception( 'Too many values remaining on the stack' );
+		}
+
+		return (int) $stack[0];
+	}
+}
diff --git a/wp-includes/pomo/translations.php b/wp-includes/pomo/translations.php
index 6ebf5f4516..1e20c37d90 100644
--- a/wp-includes/pomo/translations.php
+++ b/wp-includes/pomo/translations.php
@@ -7,6 +7,7 @@
  * @subpackage translations
  */
 
+require_once dirname(__FILE__) . '/plural-forms.php';
 require_once dirname(__FILE__) . '/entry.php';
 
 if ( ! class_exists( 'Translations', false ) ):
@@ -187,7 +188,7 @@ class Gettext_Translations extends Translations {
 	function nplurals_and_expression_from_header($header) {
 		if (preg_match('/^\s*nplurals\s*=\s*(\d+)\s*;\s+plural\s*=\s*(.+)$/', $header, $matches)) {
 			$nplurals = (int)$matches[1];
-			$expression = trim($this->parenthesize_plural_exression($matches[2]));
+			$expression = trim( $matches[2] );
 			return array($nplurals, $expression);
 		} else {
 			return array(2, 'n != 1');
@@ -201,11 +202,13 @@ class Gettext_Translations extends Translations {
 	 * @param string $expression
 	 */
 	function make_plural_form_function($nplurals, $expression) {
-		$expression = str_replace('n', '$n', $expression);
-		$func_body = "
-			\$index = (int)($expression);
-			return (\$index < $nplurals)? \$index : $nplurals - 1;";
-		return create_function('$n', $func_body);
+		try {
+			$handler = new Plural_Forms( rtrim( $expression, ';' ) );
+			return array( $handler, 'get' );
+		} catch ( Exception $e ) {
+			// Fall back to default plural-form function.
+			return $this->make_plural_form_function( 2, 'n != 1' );
+		}
 	}
 
 	/**
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 4e8f5432e7..11a62f53f4 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-alpha-41721';
+$wp_version = '4.9-alpha-41722';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.