CMSC 330, Fall 2015

Organization of Programming Languages

Project 3 - Regular Expression Interpreter

Due Mon Oct 26 11:59pm


In this project, you will write an OCaml module to implement NFAs and regular expressions.

Getting Started

  • Download the archive file and extract its contents.

Along with files used to make direct submissions to the submit server (submit.jar, .submit , submit.rb), you will find the following project files:

To test your implementation, you can execute the public tests from the command line by typing commands like ocaml, or you can use the goTest.rb script. Note that to use goTest.rb you must first edit the file and specify which diff program to use.

For this project you are allowed to use the library functions found in the Pervasives module loaded by default, as well as functions from the List and String modules. As in the previous project, you are not allowed to use imperative OCaml, except for the next function to to generate new NFA state numbers. You will receive a 0 for any functions using restricted features - we will be checking your code!

Project Description

Your job is to implement a module Nfa that includes an API for implementing both NFAs and regular expressions. The signature and starter implementation for the NFA module is provided. You may not change the NFA signature in any way, though your implementation may include more types and functions than are listed in the signature. The implementation contains a parser you can use to make it easier to test your implementation. We say more about the parser, below.

Part 1: NFAs

For the first part of this project, you will write a series of functions to implement NFAs using OCaml.

module type NFA =
    type nfa
    type transition = int * char option * int
    val make_nfa : int -> int list -> transition list -> nfa
    val e_closure : nfa -> int list -> int list
    val move : nfa -> int list -> char -> int list
    val accept : nfa -> string -> bool

Here are descriptions of the elements of this signature, and what you need to do to implement them:

  • type nfa - This is an abstract type representing NFAs. It is up to you to decide exactly how NFAs are implemented. Since the type is abstract, no client that uses your module will be able to see exactly how they are implemented.

  • type transition = int * char option * int - This is a (non-abstract) type we've made up for convenience to describe an NFA transition. In the NFAs for this project, states will simply be identified by number. Then (s0, Some c, s1) represents a transition from the state numbered s0 to the state numbered s1, via an arc labeled with the character c. Notice that the character is optional---the transition (s0, None, s1) represents an epsilon transition from s0 to s1.

  • make_nfa : int -> int list -> transition list -> nfa. This function takes as input the starting state, a list of final states, and a list of transitions, and returns an NFA. Again, it is up to you to decide exactly how NFAs should be implemented, but you probably do not need to do much more than track these three components (the starting state, final states, and transition list). As one example,
      let m = make_nfa 0 [2] [(0, Some 'a', 1); (1, None, 2)]
    sets m to be an NFA with start state 0, final state 2, a transition from 0 to 1 on character a, and an epsilon transition from 1 to 2.

  • e_closure: nfa -> int list -> int list. This function takes as input an nfa and a list of states. The output will be a list of states (in any order, with no duplicates) that the NFA might be in making zero or more epsilon transitions, starting from the list of initial states given as an argument to e_closure. For example, letting m be the NFA above, e_closure would return the following:
      e_closure m [0]     (* returns [0]   *)
      e_closure m [1]     (* returns [1;2] *)
      e_closure m [2]     (* returns [2]   *)
      e_closure m [0;1]   (* returns [0;1;2] *)

  • move : nfa -> int list -> char -> int list. This function takes as input an nfa, a list of initial states, and a character. The output will be a list of states (in any order, with no duplicates) that the NFA might be in after making one transition on the character, starting from one of the initial states given as an argument to move. For example, letting m be the NFA above, move would return the following:
      move m [0] 'a'    (* returns [1] *)
      move m [1] 'a'    (* returns [] *)
      move m [2] 'a'    (* returns [] *)
      move m [0;1] 'a'  (* returns [1] *)

    Notice that the NFA uses an implicit dead state. If s is a state in the input list and there are no transitions from s on the input character, then all that happens is that no states are added to the output list for s.

  • accept : nfa -> string -> bool. This function takes an NFA and a string, and returns true if the NFA accepts the string, and false otherwise. You will find functions in the String library to be helpful.

Note: You need to be a bit careful whenever you combine NFA representations to be sure that state names (i.e., integers) don't conflict. You might use the following internal function as an aid in this process:

  • next : unit -> int - Return a new integer, different from any values previously returned by next. (This function is defined on the OCaml slides.)

Part 2: Regular Expressions

The second part of this project is to implement regular expressions. The signature NFA contains the following declarations:

module type NFA =
    type regexp =
      | Char of char
      | Union of regexp * regexp
      | Concat of regexp * regexp
      | Star of regexp

    val regexp_to_string : regexp -> string 
    val regexp_to_nfa : regexp -> nfa

Here regexp is an user-defined OCaml variant datatype that represents regular expressions:

  • Empty_String represents the regular expression recognizing the empty string (not the empty set!). Written as a formal regular expression, this would be epsilon.

  • Char c represents the regular expression that accepts the single character c. Written as a formal regular expression, this would be c.

  • Union (r1, r2) represents the regular expression that is the union of r1 and r2. For example, Union(Char 'a', Char'b') is the same as the formal regular expression a|b.

  • Concat (r1, r2) represents the concatenation of r1 followed by r2. For example, Concat(Char 'a', Char 'b') is the same as the formal regexp ab.

  • Star r represents the Kleene closure of regular expression r. For example, Star (Union (Char 'a', Char 'b')) is the same as the formal regexp (a|b)*
For this part of the project you need to implement the following:
  • regexp_to_string : regexp -> string takes a a regular expression and returns a string for the regular expression in the postfix notation. Postfix notation is a mathematical notation in which every operator follows all of its operands. For example, the infix expression "(3 + (4 * 8)) + ((6 + 7)/5)" is expressed as "3 4 8 * + 6 7 + 5 / +". Using the same idea for regular expressions, the infix regular expression "((a|b)*aba*)*(a|b)(a|b)" can be represented as "a b | * a . b . a * . * a b | . a b | .". You can do this as a postorder DFS traversal over the regexp data structure.

  • regexp_to_nfa : regexp -> nfa takes a regexp and returns an NFA that accepts the same language as the regular expression. As long as your NFA accepts the correct language, the structure of the NFA does not matter (since the NFA produced by regexp_to_nfa will only be tested by seeing which strings it accepts).

Provided: Regular Expressions Parser

In the starter files we have provided code that parses strings into regexp values. In particular, it implements the following elements of the signature:

module type NFA =
    val string_to_regexp : string -> regexp
    val string_to_nfa : string -> nfa
    exception IllegalExpression of string
  • string_to_regexp : string -> regexp takes a string for a regular expression, parses the string, and outputs its equivalent regexp. If the parser determines that the regular expression has illegal syntax, it will raise an IllegalExpression exception.
  • string_to_nfa : string -> nfa takes a string for a regular expression, parses the string, converts it into a regexp, and transforms it to an nfa, using your regexp_to_nfa function. As such, for this functiont to work, your regexp_to_nfa function must be working.

Notes about the grammar for regular expressions implemented by the provided parser:

  • The regular expressions can contain only (, ), |, *, a, b, ..., z and E (for epsilon).
  • Note that the precedence for regular expressions are as follows, from highest to lowest:

    Precedence Operator Description
    Highest ( ) parentheses
    | * closure
    v . concatenation
    Lowest | union

  • Note that all the binary operators are right associative.
  • Your function should throw an IllegalExpression exception for invalid regular expressions.
Some examples of regular expressions and their equivalent regexp data type are listed in the following table:
    String Input regexp Output String Output
    a Char 'a' a
    a|b Union(Char 'a',Char 'b') a b |
    ab Concat(Char 'a',Char 'b') a b .
    aab Concat(Char 'a',Concat(Char 'a',Char 'b')) a a b . .
    (a|E)* Star(Union(Char 'a',Empty_String)) a E | *
    (a|E)*(a|b) Concat(Star(Union(Char 'a',Empty_String)),Union(Char 'a',Char 'b')) a E | * a b | .


You can submit your project in two ways:
  • Submit your file directly to the submit server by clicking on the submit link in the column "web submission".

    Next, use the submit dialog to submit your file.

    Select your file using the "Browse" button, then press the "Submit project!" button.

  • Submit directly by executing a Java program on a computer with Java and network access. Use the submit.jar file from the archive, To submit, go to the directory containing your project, then either execute submit.rb or type the following command directly:

    java -jar submit.jar

    You will be asked to enter your class account and password, then all files in the directory (and its subdirectories) will be put in a jar file and submitted to the submit server. If your submission is successful you will see the message:

    Successful submission # received for project 4

Academic Integrity

The Campus Senate has adopted a policy asking students to include the following statement on each assignment in every course: "I pledge on my honor that I have not given or received any unauthorized assistance on this assignment." Consequently your program is requested to contain this pledge in a comment near the top.

Please carefully read the academic honesty section of the course syllabus. Any evidence of impermissible cooperation on projects, use of disallowed materials or resources, or unauthorized use of computer accounts, will be submitted to the Student Honor Council, which could result in an XF for the course, or suspension or expulsion from the University. Be sure you understand what you are and what you are not permitted to do in regards to academic integrity when it comes to project assignments. These policies apply to all students, and the Student Honor Council does not consider lack of knowledge of the policies to be a defense for violating them. Full information is found in the course syllabus---please review it at this time.

Copyright Notice

This course project is copyright of Dr. Michael Hicks. ©Michael Hicks [2015]. All rights reserved. Any redistribution or reproduction of part or all of the contents in any form is prohibited without the express consent of the author.

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