Update README.md

Update README.md

Update README.md

Update README.md

Update README.md

Update README.md

Update README.md

Update Valid Username Regular Expression.java

Update Tag Content Extractor.java

Update Pattern Syntax Checker.java

Update Java Substring.java

Update Java Substring Comparisons.java

Update Java Strings Introduction.java

Update Java String Tokens.java

Update Java Regex.java