gistfile1.php[345]

<?php
$str1 = "/room/word:yudoufu/color:pink";
$str2 = "/room/word:yudoufu";
$str3 = "/room/color:pink";

$pattern = "#^/room(?P<params>(?:/(?:word|color)+:[^/]+)+)$#";

if (preg_match($pattern, $str1, $m)) var_dump($parameterString = $m['params']);
if (preg_match($pattern, $str2, $m)) var_dump($m['params']);
if (preg_match($pattern, $str3, $m)) var_dump($m['params']);

// string(24) "/word:yudoufu/color:pink"
// string(13) "/word:yudoufu"
// string(11) "/color:pink"

$params = array();
foreach (explode('/', trim($parameterString, '/')) as $keyValue) {
    list($k, $v) = explode(':', $keyValue);
    $params[$k] = $v;
}

var_dump($params);

// array(2) {
//   ["word"]=>
//   string(7) "yudoufu"
//   ["color"]=>
//   string(4) "pink"
// }