## Frequently Asked Questions

- Does this package use the same kind of "regular expressions" as Perl or java.util.regex?
- No. This package uses
*deterministic*finite-state automata (DFA), unlike most other regexp-packages that are based on nondeterministic automata (NFA). This means:- The notion of "regular expressions" used here has exactly the expressiveness of good old regular languages.
- This package supports operations such as complement (the
`~`operator) and intersection (the`&`operator). - The
`*`operator is mathematically the Kleene star operator (i.e. we don't have greedy/reluctant/possesive variants). - The time for pattern matching is optimal: once the regular expression has been converted into an automaton, it takes linear time in the length of a string to check whether it matches the expression - independently of the complexity of the expression. (NFA-based packages use backtracking.)
- There is no support for capturing groups.

- Can you give an example of using the package to do simple pattern matching?
- Here is a very simple example to get you started:
RegExp r = new RegExp("ab(c|d)*"); Automaton a = r.toAutomaton(); String s = "abcccdc"; System.out.println("Match: " + a.run(s)); // prints: true

- Where is the description of the syntax of regular expressions being supported by this package?
- Look in the javadoc for the
`RegExp`class. Note that this is presumably*not*exactly the same syntax as you are used to! - How do I make
__really__fast pattern matching with this package? - Convert your automaton to a
`RunAutomaton`. - How do I use the pre-built automata in my regular expressions, as in
"
`<URI>`"? - Convert your expression
`x`into an automaton using`x.toAutomaton(new DatatypesAutomatonProvider())`. Here is a list of the pre-built automata. - How do I refer to dk.brics.automaton in a publication?
- Use this BibTeX entry.
- Why is there no automata operation for [...]?
- Because there hasn't been the need so far. If you make any extensions to the package or have some suggestions for improvements, send an email to amoeller@cs.au.dk.