Regex class not working on server side of GWT

I've the following piece of code in the server folder of the module.

Pattern regex = Pattern.compile("/(B[A-Z0-9]{9})/");
Matcher matcher = regex.matcher(detailPageUrl);

But I still get the following errors:

[ERROR] Line 14: The import java.util.regex cannot be resolved
[ERROR] Line 15: The import java.util.regex cannot be resolved
[ERROR] Line 202: Pattern cannot be resolved to a type
[ERROR] Line 202: Pattern cannot be resolved
[ERROR] Line 203: Matcher cannot be resolved to a type

I've imported

import java.util.regex.Matcher;
import java.util.regex.Pattern;

in the code.

I开发者_如何学编程 thought that Regex will work on the server side. Why is this not working?

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Check out the Java environment first. Like the Java version, classpath, etc. The error clearly states that the package cannot be imported due to some reason.