For Classifier4J I use regular expressions. In the past I've used the Apache Regexp and ORO libraries, and been happy with them. However, for a first cut of the Classifier4J code I've used the JDK1.4 regular expression functionality and I quite like it. I wonder if using 1.4 only APIs is acceptable yet?

At work we are about to migrate to 1.4.1 (or 1.4.2) for development (we already deploy on 1.4 VMs), and with 1.5 to be released next year I wonder how much backwards compatibiliy is required?