为什么String.replaceAll（）在Java 8中与Java 9的工作方式不同？

Question

Why does this code output 02 in java-8 but o2 in java-9 or above?

"o2".replaceAll("([oO])([^[0-9-]])", "0$2")

Answer 1

Most likely due to JDK-6609854 and JDK-8189343 which reported negative nested character classes handling (in your example [^[0-9-]] ). This behavior was fixed in 9 and 10, but fix was not backported to 8. The bug for Java 8 is explained as:

In Java, the negation does not apply to anything appearing in nested [brackets]

So [^c] does not match "c", as you would expect.

[^[c]] does match "c". Not what I would expect.

[[^c]] does not match "c"

The same holds true for ranges or property expressions - if they're inside brackets, a negation at an out level does not affect them.

[^az] is opposite from [^[az]]