正規表現

ユニコード文字表現 \\uXXXX を HTML実体参照 &#xXXXXに変換

Perl

$str = "\\u306C\\u308B\\u307D&"
$str =~ s/\\u([0-9a-fA-F]{4})/&#x$1;/g;
print $str;

Ruby

str = "\\u306C\\u308B\\u307D"
str.gsub!(/\\u([0-9a-fA-F]{4})/) { "&#x" + $1 + ";" }
print str

subメソッドが文字列置換、gsubメソッドは、perlで言うgオプション付きの置換。!付きのメソッド（sub!、gsub!）はレシーバ自体を変更するので注意。

PHP

$str = "\\u306C\\u308B\\u307D"
$str = preg_replace("/\\\\u([0-9a-fA-F]{4})/", "&#x$1;", $str);
echo $str;

JavaScript

var str = "\\u306C\\u308B\\u307D";
str = str.replace(/\\u([0-9a-fA-F]{4})/g, function(m, p1, o, s){return "&#x" + p1 + ";"} );
document.write(str);

Java

String str = "\\u306C\\u308B\\u307D";        
Matcher m = Pattern.compile("\\\\u([0-9a-fA-F]{4})").matcher(str);
 
StringBuffer buf = new StringBuffer();
while (m.find()) {
    m.appendReplacement(buf,  "&#x$1;");
}
m.appendTail(buf);
System.out.println(buf);

String#replaceAllメソッドは後方参照可能。replaceメソッドは不可。

String str = "\\u306C\\u308B\\u307D";
str = str.replaceAll("\\\\u([0-9a-fA-F]{4})", "&#x$1;");
System.out.println(str);

C#.NET

string str = "\\u306C\\u308B\\u307D";
Regex regex = new Regex("\\u([0-9a-fA-F]{4})");
str = regex.Replace(str, "&#x$1;");
Console.WriteLine(str);