<body(.|n)+</body>

//original html
val file= File("""/home/yazan/Documents/books.xml""")

//empty file
val file2= File("""/home/yazan/Documents/books2.xml""")

val reg="""<body(.|n)+</body>""".toRegex()
val text= reg.find(file.readText())
text?.value?.let { file.writeText(it) }