How best to access the number of capturing groups in a Pattern?

Question

I want to validate the number of capturing groups in an instance of Pattern. I know method Matcher.groupCount() exists, but it requires a valid input string.

Is there a way to access the number of capturing groups without an instance of Matcher?

My current hack uses reflection to access the package-private field Pattern.capturingGroupCount, but this is terrible for many reasons.

Field f = Pattern.class.getDeclaredField("capturingGroupCount");
int capturingGroupCount = (int) f.get(pattern);

I use Java 9+, so it is not possible -- that I know -- to create a helper method in package java.util.regex to access package-private field capturingGroupCount. (The Java compiler complains that package java.util.regex is already defined in module java.base.)

Final thought / opinion: I am surprised the Java API does not expose this member, e.g., int capturingGroupCount().

Does this answer your question? [Identifying capture groups in a Regex Pattern](https://stackoverflow.com/questions/4589643/identifying-capture-groups-in-a-regex-pattern) — Booboo, Dec 01 '19 at 17:18

score 4 · Accepted Answer · answered Dec 01 '19 at 06:51

I know method Matcher.groupCount() exists, but it requires a valid input string.

No, it doesn't.

The following has been tested on Java 7 and Java 13:

Pattern p = Pattern.compile("(x)y(z)");
Matcher m = p.matcher("");
System.out.println(m.groupCount()); // prints: 2

As you can see, you don't even have to call find() or matches() first, but you do have to create the Matcher instance.

Is there a way to access the number of capturing groups without an instance of Matcher?

Not in plain Java, i.e. without using reflection.

How best to access the number of capturing groups in a Pattern?

1 Answers1