We present a formalism capable of searching and optionally replacing forests of subtrees within labelled trees. In particular, the formalism is developed to process linguistic treebanks. When used as a substitution tool, the interpreter processes rewrite rules consisting of left and right side. The left side specifies a forest of subtrees to be searched for within a tree by imposing a set of constraints encoded as a query formula. The right side contains the respective substitutions for these subtrees. In the search mode only the left side is present. The formalism is fully implemented. The performance of the implemented tool allows to process even large linguistic corpora in acceptable time. The main contribution of the presented work consists of the expressiveness of the query formula, in the elegant and intuitive way the rules are written (and their easy reversibility), and in the performance of the implemented tool.