Modifying Xpath queries dynamically during an ongoing Xpath evaluation. A modification request comprising at least one Xpath query in response to an input is received in an ongoing Xpath evaluation on an online stream of XML messages. A current generation of Nondeterministic Finite Automaton (NFA) is generated and the branches starting from the root node are identified according to the modification request. The identified branches are copied and modified to create a new generation of NFA. New generation of NFA is used for subsequent Xpath evaluations.