MinXML by back reference extension

example.txt

assert(S("<foo></foo>").drop == ParseSuccess(None, ""))
assert(S("<foo></bar>").drop == ParseFailure("", "bar>"))
assert(S("<foo><bar></bar></foo>").drop == ParseSuccess(None, ""))
assert(S("<foo><bar></foo></bar>").drop == ParseFailure("",  "<bar></foo></bar>"))

MinXML.scala

object MinXML {
  lazy val I: P[Any] = range('a'to'z','A'to'Z', Seq('_')) ~ range('a'to'z','A'to'Z',Seq('_'),'0'to'9').*
  lazy val S: P[Any] = "<" ~ I.evalCC {tag =>
     ">" ~ S.* ~ "</" ~ tag ~ ">"
  }
}