Write Your Own Regular Expression Parser

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Environment:WinXP, Win2k, WinME, Win9X, VC++6, VC++7


Have you ever wondered how regular expressions work in detail? If the answer is yes, this article is right for you. I will try to guide you step by step on how to create your own mini regular expression library. The purpose of the article is NOT to give you a highly optimized, tested, and great regular expression library, but to explain the principles behind the pattern matching in text files. If you only want a good library and don't care how it works, you probably want boost the regex library, which you can find here. I used a couple of different regular expression libraries but I must say that I am happiest with boost regex. Obviously, you decide for yourself. There is another regular expressions article that could be found here. I must admit I did not read the article completely, but it seems to me that the article focuses mostly on how to use a regular expression library provided by the author. In this article, the author uses a similar technique (I could be wrong, though) to create a more sophisticated library. The aticle you are reading right now could be seen as a prerequisite for the article I just mentioned above.

Regular expressions are part of the MS.NET library or Java SDK (if you write code in Java). As you can see, regular expressions are available in many different programming languages and technologies. Actually, the article does not focus on writing the library in a specific language. I wrote the code in C++, using STL primarily because it is my favorite language/library, but the principles from the article could be applied to any language (obviously). I will try to be as language independent as possible using pseudo code wherever possible. If you want the code in Java, please send me an e-mail. The code provided in this article is free (obviously), but if you like it and use it in your application, it would be great if you would give me the credit for what I deserve. Also please e-mail me, so I can show off to my peers and/or potential employers.


So, how are we going to do it? Well, before we start with coding, it is necessary to explain the mathematical background needed to fully understand the method used in this article. I would strongly recommend reading and understanding the math behind, because once we overcome the math part, the rest will be very simple. Note, however, that I will not have any mathematical proofs. If you are interested in proofs, please check out the references, which could be found in References section of this article. Additionally, note that the regular expression parser, which we will create here, will support these three operations:

  1. Kleen Closure or Star operator ("*")
  2. Concatenation (for example: "ab")
  3. Union operator (denoted with character "|")

However, many additional operators can be simulated by combining these three operators. For instance:

  1. A+ = AA* (At least one A)
  2. [0-9] = (0|1|2|3|4|5|6|7|8|9)
  3. [A-Z] = (A|B|...|Z), etc.

The reason for implementing only three operators is simplicity. When I started planning to write this article, I quickly recognized that I had to limit myself in many ways. The topic is so large that it would require a book to explain every little detail (maybe I write it someday). As I stated above, the purpose of the article is not to equip you with a library but to introduce you to principles behind regular expressions. If you want to know more on how to use the regular expressions, you can check out the book Mastering Regular Expressions, published by O'Reilly.

Following is the overview of the article:

  1. What Is NFA?
  2. What Is DFA?
  3. Thompson's Algorithm
  4. Subset Construction Algorithm
  5. DFA Optimization
  6. Using the Results from the Parts Above
  7. Final Words
  8. Reference

What Is NFA?

NFA stands for nondeterministic finite-state automata. NFA can be seen as a special kind of final state machines, which are in a sense an abstract model of a machine with a primitive internal memory. If you want to know more about finite-state machines, please refer to the references below.

Let us look at the mathematical definition of NFA.

An NFA A consists of:

  1. A finite set I of input symbols
  2. A finite set S of states
  3. A next-state function f from SxI into P(S)
  4. A subset Q of S of accepting states
  5. An initial state s0 from S

denoted as A(I, S, f, Q, s0)

If we explained the above definition to a 12 year old, we could say that an NFA is set S of states that are connected by function f (maybe to a smarter 12 year old). NFAs are represented in two formats: Table and Graph. For example, let us look at the table representation:




And an equivalent graph would be:

Looking at the table/graph above, we can see that there are special transitions called epsilon transitions, which is one of the special features of NFA. A transition is the event of going from one state to another. Epsilon transition is the transition from one state to another on an empty string. In other words, we are going from one state to another on no character input. For example, as we can see from table/graph above, we can go on no input from s3 to s4 and s5, which means that we have a choice. Similarly, there is a choice to go from state s0 to states s1 or s3 on character input "a". Hence the name nondeterministic, because at some point the path, which we are able to go is not unique but we have a choice. The final and accepting states are drawn double circled (or enclosed in "{}" in the table), as in, for example, s6. Once one of these states is reached, we have an accepting state, and hence an accepting character string.

In an NFA, like the mathematical definition defines, there is always a starting state. I used Graphvis, a great tool for drawing different kinds of graphs (see References), for drawing NFAs and DFAs (see later). Because Graphvis is laying out the nodes of a graph on its own, it seems to me that a starting state is always the state drawn at the top of the graph. So, we will follow that convention.

What Is DFA?

DFA stands for deterministic finite state automata. A DFA is very closely related to an NFA. As a matter of fact, the mathematical definition of NFA works for DFA, too. But, obviously there are diferences, from which we will take advantage. One big difference is that in a DFA there are no epsilon transitions. Additionally, for each state all transitions leading away from this state are done on a different input character. In other words, if we are in state s, then on input character a, there is unique transition on that character from s. Additionally, in a DFA, all states must have a valid transition on an input character. The input character here is finite set I of input symbols (as in the mathematical definition). For example, in the above graph (under DFA), the set of symbols I would be {a,b,c,d}.

Now that we understand NFAs and DFAs, we can proceed by saying that, given any NFA, there is an equivalent DFA. (You are going to have to trust me on this because I don't think it is appropriate to give you the mathematical proof of this statement.) As humans, generally it is easier for us to construct an NFA, as well as interpret what language an NFA accepts. But, why do we need the DFAs, then? Well, if we think about computers, it is very hard to "teach" them to do very well educated guesses (sometimes even we can't make smart educated guesses). And this is exactly what the computer needs to do when traversing an NFA. If we would write an algorithm that would use an NFA to check for accepting a combination of characters, it would involve backtracking to check for choices that it did not make previously. Obviously, there are regular expression parsers that work using NFAs, but they are generally slower than those that use DFAs. This is due to the fact that a DFA has a unique path for each accepting string, so no backtracking is involved. Hence, we are going to use a DFA to check whether a combination of input characters is accepted by automata.

Note: If you really want to understand both NFAs and DFAs, I would recommend that you do further reading on these topics. It is useful as an excerise to convert from one to another, to fully understand the difference and the algorithm used here to convert an NFA to a DFA (see later).

Thompson's Algorithm

Now that we have all the mathematical background that we need to understand regular expressions, we need to start thinking about what our goal is. As a first step, we realize that we need a way of going from a regular expression (such as "ab*(a|b)*") to a data structure that will be easy to manipulate and use for pattern matching. But, let us first look at the method for converting a regular expression to an NFA. Probably the most famous algorithm for doing this conversion is Thompson's algorithm. This algorithm is not most efficient, but it ensures that any regular expression (assuming that its syntax is correct) will be successfully converted to an NFA. With the help of the basic NFA as seen from figure below, we can construct any other:

Using the basic element above, we will construct three operations, which we would like to implement in our regular expression parser, as in the following:

But, how do we go from something like "(a|b)*ab" to the graph above? If we consider what we really need to do, we can see that evaluating regular expressions is similar to evaluating the arithmetic expressions. For example, if we want to evaluate R=A+B*C-D, we could do it like this:









Here, PUSH and POP are stacks and MUL, ADD, and SUB take two operands from the stack and do the corresponding operation. We could use this knowledge for constructing an NFA from a regular expression. Let's look at the sequence of operations that need to be performed to construct an NFA from the regular expression (a|b)*cd:










As we can see, it is very similar to the evaluation of the arithmetic expressions. The difference is that, in regular expressions, the star operation pops only one element from the stack and evaluates the star operator. Additionally, the concatenation operation is not denoted by any symbol, so we would have to detect it. The code provided with the article simplifies the problem by pre-processing the regular expression and inserting a character ASCII Code 0x8 when ever a concatenation is detected. Obviously, it is possible to do this "on the fly," during the evaluation of regular expression, but I wanted to simplify the evaluation as much as possible. The pre-processing does nothing else but detect a combination of symbols that would result in concatenation, as in, for example: "ab", "a(", ")a","*a","*(", ")(".

PUSH and POP operations actually work with the stack of simple NFA objects. If we would PUSH symbol "a" on the stack, the operation would create two state objects on the heap and create a transition object on symbol "a" from state 1 to state 2. Here is the portion of the code that pushes a character on the stack:

void CAG_RegEx::Push(char chInput)
  // Create two new states on the heap
  CAG_State *s0 = new CAG_State(++m_nNextStateID);
  CAG_State *s1 = new CAG_State(++m_nNextStateID);

  // Add the transition from s0->s1 on input character
  s0->AddTransition(chInput, s1);

  // Create a NFA from these two states

  // Push it onto the operand stack

  // Add this character to the input character set

As we can see, the character is converted to a simple NFA; then, the resulting NFA is added to the stack. CAG_State class is a simple class that helps us structure the an NFA as we need it. It contains an array of transitions to other states on specific characters. Epsilon transition is a transition on character 0x0. At this point, it is easy to see the structure behind NFA. An NFA (and DFA) is stored as a sequence of states (deque of CAG_State pointers). Each state has as a member all the transitions stored in a multimap. A transition is nothing else than mapping from a character to a state (CAG_State*). For detailed definition of the CAG_State class, please refer to the code.

Now, back to the conversion from a regular expression to the NFA. Now that we know how to push the NFA onto the stack, the pop operation is trivial. Just retrive the NFA from the stack and that's it. As I said earlier, an NFA table is defined to be a double-ended queue (STL container deque<CAG_State*>). In this way, we know that the first state in the array is always the starting state, while the last state is the final/accepting state. By preserving this order, we can quickly get the first and last states as well as append and prepend additional states when performing operations (such as the Star operator). Here is the code to evaulate each individual operation:

BOOL CAG_RegEx::Concat()
  // Pop two elements
  if(!Pop(B) || !Pop(A))
    return FALSE;

  // Now evaluate AB
  // Basically, take the last state from A and add an epsilon
  // transition to the first state of B.
  // Store the result into new NFA_TABLE and push it onto the stack.
  A[A.size()-1]->AddTransition(0, B[0]);
  A.insert(A.end(), B.begin(), B.end());

  // Push the result onto the stack

  return TRUE;

As we can see, the concatenation is popping two NFAs from the stack. The first NFA is changed, so that it is now a new NFA, which is then pushed on the stack. Note that we first pop the second operand. This is the case because in regular expressions the order of operands is of importance because AB != BA (not commutative).

BOOL CAG_RegEx::Star()
  // Pop one element
    return FALSE;

  // Now evaluate A*
  // Create two new states that will be inserted at each end of
  // the deque. Also take A and make an epsilon transition from
  // the last to the first state in the queue. Add an epsilon
  // transition between two new states so that the one inserted
  // at the begin will be the source and the one inserted at the
  // end will be the destination.
  CAG_State *pStartState = new CAG_State(++m_nNextStateID);
  CAG_State *pEndState   = new CAG_State(++m_nNextStateID);
  pStartState->AddTransition(0, pEndState);

  // Add an epsilon transition from start state to the first state
  // of A
  pStartState->AddTransition(0, A[0]);

  // Add an epsilon transition from A last state to end state
  A[A.size()-1]->AddTransition(0, pEndState);

  // From A last to A first state
  A[A.size()-1]->AddTransition(0, A[0]);

  // Construct a new DFA and store it onto the stack

  // Push the result onto the stack

  return TRUE;

A star operator pops a single element from the stack, changes it according to Thompson's rule (see above), and then pushes it on the stack.

BOOL CAG_RegEx::Union()
  // Pop two elements
  if(!Pop(B) || !Pop(A))
    return FALSE;

  // Now evaluate A|B
  // Create two new states, a start state and an end state.
  // Create an epsilon transition from the start state to the
  // start states of A and B.
  // Create an epsilon transition from the end states of A and B
  // to the new end state.
  CAG_State *pStartState = new CAG_State(++m_nNextStateID);
  CAG_State *pEndState   = new CAG_State(++m_nNextStateID);
  pStartState->AddTransition(0, A[0]);
  pStartState->AddTransition(0, B[0]);
  A[A.size()-1]->AddTransition(0, pEndState);
  B[B.size()-1]->AddTransition(0, pEndState);

  // Create a new NFA from A
  A.insert(A.end(), B.begin(), B.end());

  // Push the result onto the stack

  return TRUE;

The union pops two elements, makes the transformation, and pushes the result on the stack. Note that here to we have to watch for the order of the operation.

Finally, we are now able to evaulate the regular expression. If everything goes well, we will have a single NFA on the stack, which will be our resulting NFA. Here is the code, which utilizes the functions above.

BOOL CAG_RegEx::CreateNFA(string strRegEx)
  // Parse the regular expresion using a similar method to
  // evaluate arithmetic expressions.
  // But first, we will detect concatenation and insert char(8)
  // at the position where concatenation needs to occur.
  strRegEx = ConcatExpand(strRegEx);

  for(int i=0; i<strRegEx.size(); ++i)
    // Get the character
    char c = strRegEx[i];

    else if(m_OperatorStack.empty())
    else if(IsLeftParanthesis(c))
    else if(IsRightParanthesis(c))
      // Evaluate everyting in parantheses
          return FALSE;
      // Remove left paranthesis after the evaluation
      while(!m_OperatorStack.empty() &&
           Presedence(c, m_OperatorStack.top()))
          return FALSE;

  // Evaluate the rest of the operators
      return FALSE;

  // Pop the result from the stack
    return FALSE;

  // The last NFA state is always the accepting state
  m_NFATable[m_NFATable.size()-1]->m_bAcceptingState = TRUE;

  return TRUE;

Function Eval is actually evaluating the next operator on the stack. Function Eval() pops the next operator from the operator stack; and, using the switch statement, it determines which operation to use. Parantheses are treated as operators, too, because they determine the order of evaluation. The function Presedence(char Left, char Right) determines the precedence of two operators and returns TRUE if precedence of the Left operator <= precedence of the Right operator. Please check out the code for the implementation.

Subset Construction Algorithm

Now that we know how to convert any regular expression to an NFA, the next step is to convert an NFA to a DFA. At first, this process seems to be very challenging. We have a graph with zero or more epsilon transitions, and multiple transitions on a single character, and we need an equivalent graph with no epsilon transitions and unique path for each accepted sequence of input characters. Like I said, it seems to be very challanging, but it really is not. Mathematicians have actually solved that problem for us already, and then using the results, computer scientists created the Subset Construction Algorithm. I am not sure who to give credit here, but the Subset Construction Algorithm goes like this:

First, let us define two functions:

  • Epsilon Closure: This function takes as a parameter a set of states T and returns again a set of states containing all those states that can be reached from each individual state of the given set T on epsilon transition.
  • Move: Move takes a set of states T and input character a and returns all the states that can be reached on a given input character from all states in T.

Now, using these two functions, we can perform the transformation:

  1. The start state of the DFA is created by taking the epsilon closure of the start state of the NFA.
  2. For each new DFA state, perform following for each input character:
    1. Perform a move to the newly created state.
    2. Create a new state by taking the epsilon closure of the result (i). Note that here we could get a state that is already present in our set. This will result in a set of states that will form new DFA state. Note that here, from one or many NFA states, we are constructing a single DFA state.
  3. For each newly created state, perform Step 2.
  4. Accepting states of DFA are all those states that contain at least one of the accepting states from the NFA. Keep in mind that here we are constructing a single DFA state from one or many NFA states.

Simple enough? If not, read further. Following is the pseudo code found on the pages 118-121 of the book Compilers—Principles, Techniques and Tools by Aho, Sethi, and Ullman. The algorithm below is equivalent to the algorithm above but expressed in a different way. First, let's define the epsilon closure function:

S EpsilonClosure(T)
  push all states in T onto the stack
  initialize result to T
  while stack is not empty
    pop t from the stack
    for each state u with an edge from t to u labeled epsilon
      if u is not in EpsilonClosure(T)
        add u to result
        push u onto the stack
  return result

Basically, what this function does is go through all the states in T and checks what other states can be reached from these on no input. Note that each state can reach at least one state on no input, namely itself. Then, the function goes through all these resulting states and checks for further transitions on no input. For example, let us look at the following:

If we would call an epsilon transition on a set of states {s0,s2}, the resulting states would be {s0,s2,s1,s3}. This is because from s0 we can reach s1 on no input, but from s1 we can reach s3 on no input, so from s1 we can reach s3 on no input.

Now that we know how the epsilon transition works, let us look at the pseude code to transform an NFA to a DFA:

D-States = EpsilonClosure(NFA Start State) and it is unmarked
while there are any unmarked states in D-States
  mark T
  for each input symbol a
    U = EpsilonClosure(Move(T, a));
    if U is not in D-States
      add U as an unmarked state to D-States
    DTran[T,a] = U

Finally, the DTran is the DFA table, equivalent to an NFA.

Before we go to the next step, let us convert an NFA to a DFA by hand using this process. If you want to master this process, I would strongly suggest that you perform more similar transformations using this method. Let's convert following NFA to the corresponding DFA by using a subset construction algorithm:

Using the subset construction algorithm, we would do following. (Each newly created state will be bolded:)

  1. Create a start state of a DFA by taking the epsilon closure of the start state of the NFA. This step produces the set of states: {s1,s2,s4}
  2. Perform Move('a', {s1,s2,s4}), which results in set: {s3,s4}
  3. Perform EpsilonTransition({s3,s4}), which creates a new DFA state: {s3,s4}
  4. Perform Move('b', {s1,s2,s4}), which results in set: {s5}
  5. Perform EpsilonTransition({s5}), which creates new DFA state: {s5}
  6. NOTE: Here we must record two new DFA states {s3,s4} and {s5}, together with a DFA strting state {s1,s2,s4}. Also, we must record a transition on character 'a' from {s1,s2,s4} to {s3,s4} and on character b from {s1,s2,s4} to {s5}.
  7. Perform Move('a', {s3,s4}), which returns: {s4}
  8. Perform EpsilonTransition({s4}), with result: {s4}
  9. Perform Move('b', {s3,s4}), which results in set: {s3,s5}
  10. Perform EpsilonTransition({s3,s5}) with result: {s3,s5}
  11. {s3,s4}->{s4} on 'a'
  12. {s3,s4}->{s3,s5} on 'b'
  13. Perform Move('a', {s5}), which returns an empty set so we don't need to check epsilon transitions
  14. Perform Move('b', {s5}), which returns an empty set, so forget it.
  15. Perform Move('a', {s4}), which returns: {s4}. But, this is NOT a new state, so forget it. However, we must record the transition:
  16. {s4}->{s4} on 'a'
  17. Perform Move('b', {s4}) which returns: {s5}
  18. Perform EpsilonTransition({s5}) which returns: {s5} (not new, but we must record the transition)
  19. {s4}->{s5} on 'b'
  20. Perform Move('a', {s3,s5}) which returns an empty set, so forget it.
  21. Perform Move('b', {s3,s5}) which produces: {s3}
  22. EpsilonTransition({s3}) produces: {s3} a NEW DFA state
  23. {s3,s5}->{s3} on 'b'
  24. Move('a', {s3}) is an empty set
  25. Move('b', {s3}) is {s3} which is not new, but the transition must be recorded!
  26. {s3}->{s3} on 'b'

There are no new states, so we are done. Following is the drawing of the DFA:

The starting state is {s1,s2,s4}, because that is EpsilonClosure(Starting state of NFA). The Accepting states are {s5}, {s3,s4}, and {s3,s5} because they contain s3 and/or s5, which are accepting states of NFA.

DFA Optimization

Now that we have all the knowledge to convert a regular expression into an NFA and then convert NFA to an equivalent DFA, we actually could stop at this point and use it for patterns matching. Originally, when I planned to write this article, in order to keep it as simple as possible showing only principles, DFA optimization was not taken into account. But then, it occurred to me that, first of all, for large regular expressions we are creating very large NFAs (by the nature of Thompson's algorithm), which in turn occasionaly creates complex DFAs. If we were to search for patterns, this might slow us down considerably, so I decided to include the optimization as a part of the regular expression parser. The optimization here is not a complicated one. So, let's look at the following example:

Click here for a larger image.

If we look at this DFA, we notice that state 3 is first of all not a final state. Additionally, we notice that there are no outgoing edges from this state except for the loop. In other words, once we get into state 3, there is no chance to get to an accepting state. This is due to the fact that a DFA, besides the fact that it has a unique path for each accepting string and does not contain the epsilon transitions, it also must have a transition on all input characters from a particular state. (Here, the "all input characters" means the set of possibly accepting input characters. For example, in a|b|c, the set of input characters here is {a,b,c}). Here is where we abuse the math a little bit to make a DFA simpler. By deleting state 3, our DFA becomes simpler, and it still accepts same set of patterns. In this case, our DFA is not anymore exactly a DFA. If you are asking yourself why this is important, the answer is: It is not! At least for us! We will use this very basic optimization mechanism to delete all the states with such characterisitcs and so we will obtain a smaller and compacter DFA for pattern maching. To summarize, we will delete states (and all transitions leading to these states from other states) with the following characteristics:

  1. State is not accepting state
  2. State does not have any transitions to any other different state

So, the result is the following:

The DFA above definitly seems to be smaller than the the previous one. I will still call this a DFA, despite the fact that it is not really a DFA.

Using the Results from the Parts Above

Finally, we are ready to use all of the parts from above to mach some text patterns. Once we have the DFA, all we need to do is to take an input character and run it against the starting state. Here is the pseudo code to do that:

Find(string s)
  for each character c in s
    for each active state s
      if there is transition from s on c
        go to the next state t
        if t is accepting state
          record the pattern
        mark t as active
      mark s as inactive

    if there is transition from starting state on c
      go to the next state s
      if s is accepting state
        record the pattern
      mark s as active

The code above can be found in the Find(...) function of the regular expression class. To keep track of active states, I use a linked list, so I can quickly add and delete states that are active/inactive respectivly. After all the charcters are processed, all results are stored in a vector that contains pattern matches. By using the functions FindFirst(...) and FindNext(...), you can traverse trough the results. Please refer to the documentation of the CAG_RegEx class for information on how to use the class. Also, at this point I have to stress that the demo program loads a complete file into a rich edit control and then, when the searching is done, it stores it into a string, passing it as an argument to the FindFirst function. Depending on your RAM size, I would avoid loading huge files because it could take a lot of time to copy the data from one string to another, because of use of virtual memory. Like I said earlier, the program is designed to show the principle behind pattern matching in text files. Depending on time, future releases might incorporate more complete regular expression parser that searches through the files of any size and delivers the results in different ways.

At this point, for the completness of the article, I must note that there is a way of converting a regular expression directly into a DFA. This method is not explained here yet, but if the time permits, it will be in future articles (or article updates).

Final Words

Well, that's it! I hope you enjoyed reading the article as much as I enjoyed writing it. Please use the demo code in any kind of applications, but give me the credit where deserved. If you want to build a more complete library using the demo code presented here, please send me a copy of your additions.

Note: The class CAG_RegEx contains two functions—SaveDFATable() and SaveNFATable—that in debug mode save the NFA and DFA to "c:\NFATable.txt" and "c:\DFATable.txt" respectivly. As the names already reveal, these are NFA and DFA tables. Additionally, the class has functions SaveNFAGraph() and SaveDFAGraph(), which in debug mode create two files "c:\NFAGraph.dot" and "c:\DFAGraph.dot". These files are simple text files, contining the instructions for drawing these graphs using Graphviz (Check out Reference 4 below).

References & Tools Used

  1. Discrete Mathematics—Richard Johnsonbaugh (Fifth Edition)
  2. Discrete Mathematics and Its Applications—Kenneth H. Rosen (Fourth Edition)
  3. Compilers—Principles, Techniques, and Tools—Aho, Sethi, and Ullman
  4. Graphviz from ATT (tool for drawing of any kind of graphs). You can find it here: http://www.research.att.com/sw/tools/graphviz/


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