You can use PetitPreprocessor: Gofer new squeaksource3: 'PetitPreprocessor'; package: 'PetitPreprocessor'; load Now it's just transform a matching regex to a substitution String (MyParser preProcess: ('toPreprocess' asRegex) into: 'preProcessed' ) but I plan to improve it by providing a PParser to detect a matching and apply its transformation block (myParser preProcessor: (matchingParser ==> [:m | ..... ])).