src/Access/CustomParameterNamesAccessCheck.php |  2 +-
 src/JsonApiSpec.php                            | 60 +++++++++++++++-----------
 tests/src/Unit/JsonApiSpecTest.php             |  2 +-
 3 files changed, 38 insertions(+), 26 deletions(-)

diff --git a/src/Access/CustomParameterNamesAccessCheck.php b/src/Access/CustomParameterNamesAccessCheck.php
index 88f0401..5f0d082 100644
--- a/src/Access/CustomParameterNamesAccessCheck.php
+++ b/src/Access/CustomParameterNamesAccessCheck.php
@@ -51,7 +51,7 @@ class CustomParameterNamesAccessCheck implements AccessInterface {
   protected function validate(array $json_api_params) {
     foreach (array_keys($json_api_params) as $name) {
       // Ignore reserved (official) query parameters.
-      if (in_array($name, JsonApiSpec::RESERVED_QUERY_PARAMETERS)) {
+      if (in_array($name, JsonApiSpec::getReservedQueryParameters())) {
         continue;
       }
 
diff --git a/src/JsonApiSpec.php b/src/JsonApiSpec.php
index b1e6ca6..ec9cc0d 100644
--- a/src/JsonApiSpec.php
+++ b/src/JsonApiSpec.php
@@ -36,30 +36,13 @@ final class JsonApiSpec {
   const MEMBER_NAME_INNER_ALLOWED_CHARACTERS = "[a-zA-Z0-9\x{80}-\x{10FFFF}\-_ ]";
 
   /**
-   * Member name: regular expression.
+   * Checks whether the given member name is valid.
    *
    * Requirements:
    * - it MUST contain at least one character.
    * - it MUST contain only the allowed characters
    * - it MUST start and end with a "globally allowed character"
    *
-   * @see http://jsonapi.org/format/#document-member-names
-   */
-  const MEMBER_NAME_REGEXP = '/^' .
-    // First character must be "globally allowed". Length must be >=1.
-    self::MEMBER_NAME_GLOBALLY_ALLOWED_CHARACTER_CLASS . '{1}' .
-    '(' .
-      // As many non-globally allowed characters as desired.
-      self::MEMBER_NAME_INNER_ALLOWED_CHARACTERS . '*' .
-      // If lenght is >1, then it must end in a "globally allowed" character.
-      self::MEMBER_NAME_GLOBALLY_ALLOWED_CHARACTER_CLASS . '{1}' .
-    // >1 characters is optional.
-    ')?' .
-    '$/u';
-
-  /**
-   * Checks whether the given member name is valid.
-   *
    * @param string $member_name
    *   A member name to validate.
    *
@@ -68,17 +51,40 @@ final class JsonApiSpec {
    * @see http://jsonapi.org/format/#document-member-names
    */
   public static function isValidMemberName($member_name) {
-    return preg_match(JsonApiSpec::MEMBER_NAME_REGEXP, $member_name) === 1;
+    // @todo When D8 requires PHP >=5.6, move to a MEMBER_NAME_REGEXP constant.
+    static $regexp;
+    if (!isset($regexp)) {
+      $regexp = '/^' .
+        // First character must be "globally allowed". Length must be >=1.
+        self::MEMBER_NAME_GLOBALLY_ALLOWED_CHARACTER_CLASS . '{1}' .
+        '(' .
+          // As many non-globally allowed characters as desired.
+          self::MEMBER_NAME_INNER_ALLOWED_CHARACTERS . '*' .
+          // If lenght is >1, then it must end in a "globally allowed" character.
+          self::MEMBER_NAME_GLOBALLY_ALLOWED_CHARACTER_CLASS . '{1}' .
+        // >1 characters is optional.
+        ')?' .
+        '$/u';
+    }
+
+    return preg_match($regexp, $member_name) === 1;
   }
 
   /**
    * The reserved (official) query parameters.
+   *
+   * @todo When D8 requires PHP >= 5.6, convert to an array.
    */
-  const RESERVED_QUERY_PARAMETERS = [
-    'sort',
-    'page',
-    'filter',
-  ];
+  const RESERVED_QUERY_PARAMETERS = 'sort|page|filter';
+
+  /**
+   * Gets the reserved (official) JSON API query parameters.
+   *
+   * @return string[]
+   */
+  public static function getReservedQueryParameters() {
+    return explode('|', static::RESERVED_QUERY_PARAMETERS);
+  }
 
   /**
    * Checks whether the given custom query parameter name is valid.
@@ -86,6 +92,12 @@ final class JsonApiSpec {
    * A custom query parameter name must be a valid member name, with one
    * additional requirement: it MUST contain at least one non a-z character.
    *
+   * Requirements:
+   * - it MUST contain at least one character.
+   * - it MUST contain only the allowed characters
+   * - it MUST start and end with a "globally allowed character"
+   * - it MUST contain at least none a-z (U+0061 to U+007A) character
+   *
    * It is RECOMMENDED that a hyphen (U+002D), underscore (U+005F) or capital
    * letter is used (i.e. camelCasing).
    *
diff --git a/tests/src/Unit/JsonApiSpecTest.php b/tests/src/Unit/JsonApiSpecTest.php
index 2fa1716..7097b35 100644
--- a/tests/src/Unit/JsonApiSpecTest.php
+++ b/tests/src/Unit/JsonApiSpecTest.php
@@ -37,7 +37,7 @@ class JsonApiSpecTest extends UnitTestCase {
     $data['space-end'] = ['kittens ', FALSE];
 
     // Additional test cases.
-    $data['unicode-above-u+0080-highest-allowed'] = ["12\u{10FFFF}", TRUE];
+    $data['unicode-above-u+0080-highest-allowed'] = ["12􏿿", TRUE];
     $data['single-character'] = ['a', TRUE];
 
     $unsafe_chars = [