This paper introduces to the finite-state calculus a family of directed replace operators. In contrast to the simple replace expression, UPPER -> LOWER, defined in Karttunen (1995), the new directed version, UPPER @-> LOWER, yields an unambiguous transducer if the lower language consists of a single string. It transduces the input string from left to right, making only the longest possible replacement at each point. A new type of replacement expression, UPPER @-> PREFIX … SUFFIX, yields a transducer that inserts text around strings that are instances of UPPER. The symbol … denotes the matching part of the input which itself remains unchanged. PREFIX and SUFFIX are regular expressions describing the insertions. Expressions of the type UPPER @-> PREFIX … SUFFIX may be used to compose a deterministic parser for a ‘local grammar’ in the sense of Gross (1989). Other useful applications of directed replacement include tokenization and filtering of text streams.

