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.
- Some symbols, like ^ and $, may mean something else than you expect.
- 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.
- How do I report bugs or suggest improvements?
- Please use the github issue tracker, or send an email to amoeller@cs.au.dk.