Regular expression to parse Java package and class name from cannonical class name

Question

I'm configuring a log parsing system (Logstash) that uses Regular expressions to parse logs. I'm trying to parse out a package name and class name from a canonical (i.e. fully qualified) Java class name, but I can't get it quite right.

Here are some sample inputs

1. UnpackagedClass
2. somepackage.SomeClass
3. java.lang.Object
4. java.util.function.Function

Expected output (capture groups):

1. (empty string), UnpackagedClass
2. somepackage, SomeClass
3. java.lang, Object
4. java.util.function, Function

Here is what I tried: ((?:(?:X)\.)*)((?:X)), where X is [a-zA-Z_$][a-zA-Z\d_$]*, the regex for a Java identifier. Fully expanded, it's: ((?:(?:[a-zA-Z_$][a-zA-Z\d_$]*)\.)*)((?:[a-zA-Z_$][a-zA-Z\d_$]*)). It's close, but there are trailing periods after the package names, that get captured as part of the package names:

1. (empty string), UnpackagedClass
2. somepackage., SomeClass
3. java.lang., Object
4. java.util.function., Function

Any suggestions on how I can improve this? Here's a RegExr playground to help you 😊.

Answer 1

Use: (?:(X(?:\.X)*)\.)?(X)

It will have package name in group 1 (null if unnamed), and class name in group 2.

See regex101.com for demo.