1. A system for retrieving documents related to a concept from a text corpus comprising: a memory which stores: a set of semantic classes which are combinable to express the concept, a set of keywords for each of the semantic classes to be used in searching documents in the text corpus, each set of keywords including at least one keyword, and a plurality of syntactic rules to be applied to identified text portions which include one or more of the keywords, each of the syntactic rules identifying a first of the semantic classes and a second of the semantic classes, the rule being satisfied when a keyword from the first of the semantic classes and a keyword from the second of the semantic classes are in any one of a plurality of syntactic relationships; and a concept matching module, which accesses the memory, for identifying text portions within the text corpus which include one or more of the keywords and for applying the syntactic rules to the text portions and identifying those text portions which satisfy at least one of the syntactic rules, whereby documents are retrieved which include at least one of the identified text portions.