Xquery to mask CreditCard number

declare function local:maskString($input as xs:string, $numClearDigits as xs:int)

as xs:string{

let $len := fn:string-length($input)

let $maskLen := $len - $numClearDigits

let $clearText := fn:substring($input, $maskLen+1)

let $mask := fn:string-join (

(for $i in (1 to $maskLen) return "X", $clearText)

,'')

return

$mask

};