1. A system comprising: a processor; a lexical data store having a plurality of lexical entries, each lexical entry having a textual string and one or more segment identifiers that are mapped to the textual string; a rules data store having a plurality of rule entries, each rule entry representing a rule that indirectly identifies valid compound words by representing relationships between segment identifiers in the lexical data store; and executable instructions, which, when executed by the processor, configure the system to: receive an input signal indicative of a textual input; and apply a given rule from the rules data store to determine whether the textual input comprises a valid compound word.