A system and method for building merged events from log entries received from multiple devices. Multiple log events generally contribute to a single merged event. In the described embodiment, the mapping module receives log entries associated with specific merged events and maps them to fields in the merged event data structure in accordance with mapping properties. The described embodiments of the invention use regular expressions in the merge properties to describe values that are searched for in the received log entries. A described embodiment of the present invention gives the mapping module access to the event under construction. A new conditional operator, _oneOf, is introduced that selects the first token that is bound to a value out of a list of tokens.