A generalised system for language analysis and local modification is described.The analysis can be in terms of any a priori unit of the language;such as a statement in the FORTRAN language. A description of the language.is provided as input data from which the system constructs a tree—structured hierarchy. This hierarchy then directs and controls the subsequent analysis and possible modification of particular units in the given language.At the same time the system collects detailed information on the structure of all units analysed. Periodically,the normal process of analysis is suspended, and the analytic information stored within the tree—structured hierarchy is utilised by mechanisms designed on heuristic principles to optimise the subsequent processing.The optimisation derives both from modifications to the basic tree—structured hierarchy and also insertion of extra control information within this hierarchy. This results in an increased processing rate for the units analysed and an improved ability to deal with errors,including 'errors' which are definitionally correct but very 'unlikely'. The analyser is applied as a recogniser for statements written in the FORTRAN language by programmers working in industrial and university enviroments,comprising a 'normal' computer workload in those enviroments,and the results of the analysis are presented and discussed.