9. A computer implemented system least partially implemented in hardware, comprising: an Xpath parser configured to perform a first Xpath evaluation on a stream of Extensible Markup Language (XML) messages using a processor, the first Xpath evaluation based on a group of Xpath queries associated with a first user transaction, the Xpath parser configured to receive a plurality of requests to modify the Xpath query group and combining the plurality of requests into a single atomic modification; and a modifying module configured to modify an Xpath query group, in a persistent manner, without affecting the ongoing Xpath evaluation on the XML messages, wherein the modifying module is further configured to generate a second data structure as a copy of a portion of a first data structure, the first data structure comprising a hash table with branches, the branches derived from the group of Xpath queries prior to modification and only copying branches of the first data structure being modified, the modifying module configured to alter at least one branch of the second data structure in accordance with the modification request, wherein the single atomic modification modifies the second data structure to perform a subsequent Xpath evaluation corresponding to a subsequent modification request of the plurality of requests without generating an additional data structure, the modifying module configured to perform a second Xpath evaluation, associated with a second user transaction, using the second data structure in substantially real time with the first Xpath evaluation such that the first and second user transactions are overlapping, and deleting the first data structure upon completion of the first user transaction and using the second data structure for additional user transactions until modified.
9. A computer implemented system least partially implemented in hardware, comprising: an Xpath parser configured to perform a first Xpath evaluation on a stream of Extensible Markup Language (XML) messages using a processor, the first Xpath evaluation based on a group of Xpath queries associated with a first user transaction, the Xpath parser configured to receive a plurality of requests to modify the Xpath query group and combining the plurality of requests into a single atomic modification; and a modifying module configured to modify an Xpath query group, in a persistent manner, without affecting the ongoing Xpath evaluation on the XML messages, wherein the modifying module is further configured to generate a second data structure as a copy of a portion of a first data structure, the first data structure comprising a hash table with branches, the branches derived from the group of Xpath queries prior to modification and only copying branches of the first data structure being modified, the modifying module configured to alter at least one branch of the second data structure in accordance with the modification request, wherein the single atomic modification modifies the second data structure to perform a subsequent Xpath evaluation corresponding to a subsequent modification request of the plurality of requests without generating an additional data structure, the modifying module configured to perform a second Xpath evaluation, associated with a second user transaction, using the second data structure in substantially real time with the first Xpath evaluation such that the first and second user transactions are overlapping, and deleting the first data structure upon completion of the first user transaction and using the second data structure for additional user transactions until modified. 10. The computer implemented system of claim 9 , wherein said modifying module comprises: an identification module for identifying, in a current generation of Nondeterministic Finite Automaton (NFA) said current generation of NFA comprising compiled Xpath query group maintained as a data structure, at least one branch of said data structure starting from root node according to the modification request; and a copying module for creating a copy of the identified at least one branch of the data structure.