Using Java Regex to parse a string containing an unknown number of matches into a List

Question

I essentially have a String representation of a comma separated list. However, each individual element is also comma separated, so the String was modified to have each element surrounded by '<' and '>'. I am trying to use regex to capture each element and add it to a list, thus making it a List of elements, rather than a String of a List.

Here are some example String inputs:

"<>"         // should match regex, but will be thrown out
"<a=1>"
"<a=1,b=1>"
"<a=1,b=1>,<a=2,b=2>"
"<a=1,b=1>,<a=2,b=2>,<a=3,b=3,c=3>,<a=4>"

The corresponding outputs I would like would be lists like so:

["a=1"]
["a=1,b=1"]
["a=1,b=1","a=2,b=2"]
["a=1,b=1","a=2,b=2","a=3,b=3,c=3","a=4"]

The pattern I am trying to use is:

Pattern pattern = Pattern.compile("<([^>]*)>(,<([^>]*)>)*");

But when I try to create the list, it is not handling each additional occurrence as a new group.

Matcher matcher = pattern.matcher(myString);
if (matcher.matches()) {
    List<String> listOfElements = new ArrayList<>();
    for (int i = 1; i <= matcher.groupCount(); i++) { // group 0 represents the entire String, so start at index 1
        if (matcher.group(i) != null) {
            listOfElements.add(matcher.group(i));
        }
    }
    System.out.println(listOfElements);
}

The result of the above test cases are:

["a=1"]
["a=1,b=1"]
["a=1,b=1", ",<a=2,b=2>", "a=2,b=2"]
["a=1,b=1", ",<a=4>", "a=4"]

Note: I added the quotes to that result for readability to separate out the values in the list - obviously the System.out.println() does not write out the quotes.

What is the proper regex to do this? Or if there is a better way than using regex, I'd be happy to hear, though keep in mind that I'd prefer to not have to use a third party package.

Can you maybe just drop the initial < and the final >, then split the rest on >,< ? That'll give you what you want in an array, and you can then use Arrays.asList. — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Jun 5, 2018 at 23:39
@DawoodibnKareem ha, well that does indeed work, but seems like too much of a hack — Sean
– Sean, Commented Jun 5, 2018 at 23:54

Leo Aso · Accepted Answer · 2018-06-05 23:47:24Z

2

Match the entries one by one with find instead of matches.

Pattern pattern = Pattern.compile("<([^>]*)>");
Matcher matcher = pattern.matcher(myString);
List<String> listOfElements = new ArrayList<>();

while (matcher.find()) {
    listOfElements.add(matcher.group(1));
}
System.out.println(listOfElements);

edited Jun 5, 2018 at 23:47

answered Jun 5, 2018 at 23:40

Leo Aso

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2 Comments

Sean Over a year ago

This is definitely close to what I'm looking for, but the return values are keeping the brackets for some reason (maybe since it's just group()?). I could technically just replace the brackets with nothing, but there's prob a better way

Leo Aso Over a year ago

@Sean Made a typo. Try it again.

Bohemian · Accepted Answer · 2018-06-05 23:50:37Z

1

You can do it in one line by splitting using look arounds:

String[] parts = str.split("(?<=>),(?=<)");

The regex splits on commas that are preceded by > and followed by <, without consuming the angle brackets.

If you really need a List:

List<String> parts = Arrays.asList(str.split("(?<=>),(?=<)"));

edited Jun 5, 2018 at 23:50

answered Jun 5, 2018 at 23:44

Bohemian♦

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1 Comment

Sean Over a year ago

sure enough, that works. I like the regex that Leo gave a little better, but it's still good to know about this way! thanks

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