Java NIO, by Ron Hitchens (O’Reilly), shows regular expressions in the context of Java’s new I/O improvements.

Mastering Regular Expressions, Second Edition, by Jeffrey E. F. Friedl (O’Reilly), covers the details of Java regular expressions on pages 378-391.

Sun’s online documentation at http://java.sun.com/j2se/1.4/docs/api/java/util/regex/package-summary.html.