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https://issues.apache.org/jira/browse/LUCENE-7921?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16119523#comment-16119523
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Thomas Poppe commented on LUCENE-7921:
--------------------------------------
It's the opposite: unrolling gets you the benefit. I was hoping more for the
conclusion that none of the cases should be throwing the exception - as the
regexp is not that complex, and neither is the resulting automaton.
Elasticsearch has no problems executing it (in the unrolled variant).
> More efficient way to transform a RegExp to an Automaton
> --------------------------------------------------------
>
> Key: LUCENE-7921
> URL: https://issues.apache.org/jira/browse/LUCENE-7921
> Project: Lucene - Core
> Issue Type: Improvement
> Affects Versions: 6.5.1
> Reporter: Thomas Poppe
> Priority: Minor
>
> Consider the following example:
> {code:title=ToAutomatonExample.java|borderStyle=solid}
> public static void main(String[] args) {
> org.apache.lucene.util.automaton.RegExp regExp =
> new
> org.apache.lucene.util.automaton.RegExp("[a-z]{1,13}x[a-z][a-z]?[a-z]?[a-z]?[a-z]?[a-z]{0,8}");
> org.apache.lucene.util.automaton.Automaton automaton =
> regExp.toAutomaton();
> System.out.println("states: " + automaton.getNumStates());
> System.out.println("transitions: " + automaton.getNumTransitions());
> System.out.println("-------------------------------");
> try {
> regExp = new
> org.apache.lucene.util.automaton.RegExp("[a-z]{1,13}x[a-z]{1,13}");
> automaton = regExp.toAutomaton();
> System.out.println("Will not happen...");
> } catch
> (org.apache.lucene.util.automaton.TooComplexToDeterminizeException e) {
> automaton = regExp.toAutomaton(1_000_000);
> System.out.println("states: " + automaton.getNumStates());
> System.out.println("transitions: " +
> automaton.getNumTransitions());
> System.out.println("-------------------------------");
> }
> }
> {code}
> Both regular expressions are equivalent, but it's much more efficient to
> "unroll" the repetition. It might be possible to optimize the
> Regex#toAutomaton() method to handle this repetition without going over the
> default number of determinized states, and using less memory and CPU?
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