Java Regex Matching Groups Bug?

Question

I have written a basic CLI using swing components and am using Regex to recognise commands.

I have happened across something peculiar which I am at a loss to explain. What am I doing wrong here?

This is the code I have:

class GraphCommandFactory {

private final GraphSearchController controller;
private final GraphSearchModel model;
private final ArrayList<Pattern> commands;

public GraphCommandFactory(GraphSearchController controller, GraphSearchModel model) {
    this.model = model;
    this.controller = controller;
    this.commands = new ArrayList<>();

    commands.add(Pattern.compile("SET START ([A-Z]{4,8})"));
}

public Command createCommand(String commandString) {
    Command returnCommand;

    // Test the string against each regex
    int command = 0;
    Matcher matcher = commands.get(command).matcher(commandString);
    ...

private String[] extractArguments(Matcher matcher) {
    String[] arguments = new String[matcher.groupCount()];

    for (int i = 0, j = matcher.groupCount(); i < j; i++) {
        arguments[i] = matcher.group(i);
    }

    return arguments;
}

The problem comes with the extractArguments function... Using the pattern (in the Matcher):

Pattern.compile("SET START ([A-Z]{4,8})"));

loses the last group. However, if I amend it to:

Pattern.compile("SET START ([A-Z]{4,8})()"));

Then it correctly captures what I want.

Have I misunderstood how regexes, Pattern and Matcher are to be used? Or is this a bug where the last capturing group is simply lost?

I am using Java SDK 1.8 and Netbeans as my IDE. Using the debug facility leaves me none-the-wiser.

anubhava · Accepted Answer · 2016-02-20 10:20:19Z

7

Problem is in your for loop:

for (int i = 0, j = matcher.groupCount(); i < j; i++) {
    arguments[i] = matcher.group(i);
}

As you are only looping up to 1 less than the matcher.groupCount

Change this to:

for (int i = 0; i <= matcher.groupCount(); i++) {
    arguments[i] = matcher.group(i);
}

As per Javadoc:

groupCount returns the number of capturing groups in this matcher's pattern. Group zero denotes the entire pattern by convention. It is not included in this count.

answered Feb 20, 2016 at 10:20

anubhava

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2 Comments

swshaun Over a year ago

Thank you! Force of habit. I also need to add an extra array slot for String[] arguments as I want to retain the original command in case of failure.

MJar Over a year ago

It could be also changed to arguments[i] = matcher.group(i+1); while preserving the existing for for (int i = 0, j = matcher.groupCount(); i < j; i++) { because OP is rather not interested in extracting the whole matched expression but just the argument in it.

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