Java – Matcher returns matches on a regex pattern, but split() fails to find a match on the same regex

javaregex

I can't see a reason why the Matcher would return a match on the pattern, but split will return a zero length array on the same regex pattern. It should return something — in this example I'm looking for a return of 2 separate strings containing "param/value".

public class MyClass {

    protected Pattern regEx = "(([a-z])+/{1}([a-z0-9])+/?)*";

    public void someMethod() {
        String qs = "param/value/param/value";
        Matcher matcherParamsRegEx = this.regEx.matcher(qs);
        if (matcherParamsRegEx.matches()) { // This finds a match.
            String[] parameterValues = qs.split(this.regEx.pattern()); // No matches... zero length array.
        }
    }
}

Best Answer

The pattern can match the entire string. split() doesn't return the match, only what's in between. Since the pattern matches the whole string that only leaves an empty string to return. I think you might be under a misconception as to what split() does.

For example:

String qs = "param/value/param/value";
String pieces = qs.split("/");

will return an array of 4 elements: param, value, param, value.

Notice that what you search on ("/") isn't returned.

Your regex is somewhat over-complicated. For one thing you're using {1}, which is unnecessary. Second, when you do ([a-z])+ you will capture exactly one latter (the last one encountered. Compare that to ([a-z]+), which will capture the entire match. Also, you don't even need to capture for this. The pattern can be simplified to:

protected Pattern regEx = Pattern.compile("[a-z]+/([a-z0-9]+/?)*");

Technically this:

protected Pattern regEx = "(([a-z])+/{1}([a-z0-9])+/?)*";

is a compiler error, so what you actually ran versus what you posted could be anything.

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