A Type System for Regular Expressions

Regular expressions are used to match and extract text. It is easy for developers to make syntactic mistakes when writing regular expressions, because regular expressions are often complex and different across programming languages. Such errors result in exceptions at run time, and there is currently no static support for preventing them. This paper describes practical experience designing and using a type system for regular expressions. This type system validates regular expression syntax and capturing group usage at compile time instead of at run time - ensuring the absence of PatternSyntaxExceptions from invalid syntax and IndexOutOfBoundsExceptions from accessing invalid capturing groups.