Grammar Type for String

Strings are ubiquitous in computer programs. Both the correctness and the security of programs that use Strings often rely on them not being arbitrary Strings, but belonging to specific sets of Strings. However, this restriction is often not enforced, let alone clearly specified. To remediate this issue, this thesis creates a language extension on top of the standard Java language. The language extension introduces a Grammar Type that is a subtype of String but conforming to the regex expression specified for each Grammar Type. For example, String[[“a∗b”]] is a Grammar Type that represents all Strings that are any number of a’s followed by a b. When we declare or cast a variable as String[[some regex]], the variable has to be a String conforming to that some regex, otherwise it would be either a compile time error or runtime error depending on the situation. This makes the Java type system more powerful, as we can now inherently validate Strings with any pattern we define using a regex. At the same time, since Grammar Types compile down to Strings, they inherit all the functionality of Strings. With these advantages, Grammar Types can be used in various types of applications that need input validation, like validating email addresses, validating URLs, or mitigating SQL injection attacks.