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GitHub - google/libphonenumber
Google's common Java, C++ and JavaScript library for parsing, formatting, and validating international phone numbers.
Java lernen durch Ausprobieren!
LearnJ - ein interaktiver Java-Kurs für Schüler/innen
RegexPlanet: online regular expression testing for Java
Online regular expression testing for Java using java.util.regex.Pattern
Edabit
Learn to code with interactive challenges. It's the fastest, easiest, most addictive way to learn.
Sourcetrail - The open-source cross-platform source explorer
Sourcetrail uses static analysis to provide code search and dependency visualization that lets you understand, refactor and maintain unfamiliar source code.