Objective is to create DFA from a regular expression and using "Regular exp>NFA>DFA conversion" is not an option. How should one go about doing that?

I asked this question to our professor but he told me that we can use intuition and kindly refused to provide any explanation. So I wanted to ask you.

"Regular exp>NFA>DFA conversion" is not an option because such a conversion takes a lot of time to convert a rather complex regular expression. For example, for a certain regex "regex>NFA>DFA" takes 1 hour for a human being. I need to convert regex to DFA in less than 30 minutes.

  • 2
    $\begingroup$ You need to provide more context. What (informal) algorithm are you currently using to translate regular expressions? It might be helpful to explain your process with an example such as a(a|ab|ac)*a+. You can either directly translate that to an NDFA which you reduce to a DFA, or you can normalize it to something that maps immediately to a DFA. $\endgroup$
    – amon
    Mar 27, 2015 at 11:03
  • $\begingroup$ Do you have to do it on specific examples by whatever means, or do you have to provide a general procedure.to be applied by a computer? $\endgroup$
    – babou
    Mar 28, 2015 at 16:40

3 Answers 3


Since you want "to convert regex to DFA in less than 30 minutes", I suppose you are working by hand on relatively small examples.

In this case you can use Brzozowski's algorithm $[1]$, which computes directly the Nerode automaton of a language (which is known to be equal to its minimal deterministic automaton). It is based on a direct computation of the derivatives and it also works for extended regular expressions allowing intersection and complementation. The drawback of this algorithm is that it requires to check the equivalence of the expressions computed along the way, an expensive process. But in practice, and for small examples, it is very efficient.

Left quotients. Let $L$ be a language of $A^*$ and let $u$ be a word. Then $$ u^{-1}L = \{v \in A^* \mid uv \in L \} $$ The language $u^{-1}L$ is called a left quotient (or left derivative) of $L$.

Nerode automaton. The Nerode automaton of $L$ is the deterministic automaton $\mathcal{A}(L) = (Q, A, \cdot, L, F)$ where $Q = \{u^{-1}L \mid u \in A^*\}$, $F = \{u^{-1}L \mid u \in L\}$ and the transition function is defined, for each $a \in A$, by the formula $$ (u^{-1}L)\cdot a = a^{-1}(u^{-1}L)=(ua)^{-1}L $$ Beware of this rather abstract definition. Each state of $\mathcal{A}$ is a left quotient of $L$ by a word, and hence is a language of $A^*$. The initial state is the language $L$, and the set of final states is the set of all left quotients of $L$ by a word of $L$.

Brzozowski's algorithm. Let $a, b$ be letters. One can compute the left quotients using the following formulas: \begin{align*} a^{-1}1 &= 0 & a^{-1}b &= \begin{cases} 1 &\text{if $a = b$}\\ 0 &\text{if $a \not= b$}\\ \end{cases}\\ a^{-1}(L_1 \cup L_2) &= a^{-1}L_1 \cup u^{-1}L_2,& a^{-1}(L_1 \setminus L_2) &= a^{-1}L_1 \setminus u^{-1}L_2,\\ a^{-1}(L_1 \cap L_2) &= a^{-1}L_1 \cap u^{-1}L_2, & a^{-1}L^* &= (a^{-1}L)L^* \end{align*} \begin{align*} a^{-1}(L_1L_2) &= \begin{cases} (a^{-1}L_1)L_2 &\text{si $1 \notin L_1$,}\\ (a^{-1}L_1)L_2 \cup a^{-1}L_2 &\text{si $1 \in L_1$}\\ \end{cases}\\ %\\v^{-1}(u^{-1}L) &= (uv)^{-1}L. \end{align*}

Example. For $L = (a(ab)^*)^* \cup (ba)^*$, we get successively: \begin{align*} 1^{-1}L &= L=L_1\\ a^{-1}L_1 &=(ab)^*(a(ab)^*)^*=L_2\\ b^{-1}L_1 &= a(ba)^*=L_3\\ a^{-1}L_2 &= b(ab)^*(a(ab)^*)^* \cup (ab)^*(a(ab)^*)^*=bL_2 \cup L_2=L_4\\ b^{-1}L_2 &=\emptyset \\ a^{-1}L_3 &=(ba)^*=L_5\\ b^{-1}L_3 &=\emptyset \\ a^{-1}L_4 &= a^{-1}(bL_2 \cup L_2)=a^{-1}L_2=L_4 \\ b^{-1}L_4 &= b^{-1}(bL_2 \cup L_2)= L_2\cup b^{-1}L_2 = L_2 \\ a^{-1}L_5 &= \emptyset\\ b^{-1}L_5 &=a(ba)^*=L_3 \end{align*} which gives the following minimal automaton. The minimal automaton

$[1]$ J. Brzozowski, Derivatives of Regular Expressions, J.ACM 11(4), 481–494, 1964.

Edit. (April 5, 2015) I just discovered that a similar question: What algorithms exist for construction a DFA that recognizes the language described by a given regex? was asked on cstheory. The answer partly addresses complexity issues.

  • $\begingroup$ Can you say more about the complexity of this algorithm? $\endgroup$
    – babou
    Mar 28, 2015 at 20:30
  • $\begingroup$ @babou Converting a RE to a DFA is PSPACE-hard, so it's definitely exponential. $\endgroup$ Mar 29, 2015 at 21:40
  • $\begingroup$ This should probably go into the answer. The OP starts with "the standard constructions via NFA are too slow" and part of the answer seems to be "bad luck, there isn't really a fast solution". It remains to discuss whether this here is better than the standard construction. (cc @jmite) $\endgroup$
    – Raphael
    Mar 31, 2015 at 7:54
  • $\begingroup$ @jmite Yes I was expecting that. The reason for my question is why this way of building the DFA should then be considered easier. (note: the system took a full day to notify me of @ jmite answer). $\endgroup$
    – babou
    Mar 31, 2015 at 8:26

J.-E. Pin provides the better answer in terms of formality and completeness, but I think there's something to be said for the "intuition" that your professor is hinting at.

In most of these cases, the easiest thing to do is to look at a regular expression, understand what language it's accepting, then use your creativity/cleverness to construct a DFA accepting that language.

There's no straightforward way to do this, other than the algorithms others have given, but here are some guidelines that might prove useful.

  1. Ask yourself, could I write a program that accepts this RE using only boolean or very small integer variables? Then write that program, and convert it into a DFA where there's a state for every combination of values.

  2. Look for parts of the regular expression that you know you can accept deterministically, where you know "If I see this, then I must be matching this part of the RE." There won't always be tons of these, but identifying these parts can show the parts that will be easy to make a DFA, so you can spend more time on the parts that really require non-determinism.

  3. The subset construction for NFA->DFA isn't actually that complicated of an algorithm. So if this is an assignment, not an exam question, it might be faster to just code up an implementation, and let your program convert NFA to DFA. If you used your own code, there shouldn't be any plagarism issues.

Remember that no matter what you do, any technique will blow up exponentially in the worst case (unless you find a polynomial algorithm for this, in which case, congratulations, you've proved $P=NP=PSPACE$ and you're now a millionaire.)

Try to "look ahead," cut corners when you can use your intuition in places when the algorithm would require many steps but its result is clear.


Though this is not the correct way but it works most of the time.

First Step : Find the smallest string that can be accepted by the Regular Expression. Second Step : Draw necessary states with the transaction of the minimum string accepting machine. Third Step : For all states draw the remaining alphabets transactions.

For Example : Regular Expression (0 + 1)* 1 "String ending with 1" Step 1: Smallest String : 1 Step 2: two states Q0 and Q1. having the transaction of 1 from Q0 to Q1. and Q1 is the accepting state. Step 3: for Q0 State Q0 1 transaction is to Q1. Now make 0 transaction in Q0 itself. For Q1 State Q1 1 transaction will remain in Q1. And 0 transaction will go in Q0.


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