diff --git a/src/Composer/Json/JsonManipulator.php b/src/Composer/Json/JsonManipulator.php
index 40c0c09a2..8fe6a9f0a 100644
--- a/src/Composer/Json/JsonManipulator.php
+++ b/src/Composer/Json/JsonManipulator.php
@@ -22,7 +22,7 @@ class JsonManipulator
     private static $DEFINES = '(?(DEFINE)
        (?<number>   -? (?= [1-9]|0(?!\d) ) \d+ (\.\d+)? ([eE] [+-]? \d+)? )
        (?<boolean>   true | false | null )
-       (?<string>    " ([^"\\\\]* | \\\\ ["\\\\bfnrt\/] | \\\\ u [0-9a-f]{4} )* " )
+       (?<string>    " ([^"\\\\]* | \\\\ ["\\\\bfnrt\/] | \\\\ u [0-9A-Fa-f]{4} )* " )
        (?<array>     \[  (?:  (?&json) \s* (?: , (?&json) \s* )*  )?  \s* \] )
        (?<pair>      \s* (?&string) \s* : (?&json) \s* )
        (?<object>    \{  (?:  (?&pair)  (?: , (?&pair)  )*  )?  \s* \} )