Plural_Forms::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[] = array( 'var' );
						$pos++;
						break;

					// Parentheses.
					case '(':
						$stack[] = $next;
						$pos++;
						break;

					case ')':
						$found = false;
						while ( ! empty( $stack ) ) {
							$o2 = $stack[ count( $stack ) - 1 ];
							if ( '(' !== $o2 ) {
								$output[] = array( '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[] = array( '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[] = array( '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[] = array( 'value', intval( substr( $str, $pos, $span ) ) );
							$pos     += $span;
							break;
						}

						throw new Exception( sprintf( 'Unknown symbol "%s"', $next ) );
				}
			}

			while ( ! empty( $stack ) ) {
				$o2 = array_pop( $stack );
				if ( '(' === $o2 || ')' === $o2 ) {
					throw new Exception( 'Mismatched parentheses' );
				}

				$output[] = array( 'op', $o2 );
			}

			$this->tokens = $output;
		}