Furthermore the 5.8 engine is pretty much the 5.6 engine, which was significantly slower than the 5.005 engine. Why the slow-down? Because 5.6 has logic to check when regular expressions are matching slowly, and then to add tracing when it is. This makes many of the exponential slowdowns that Mastering Regular Expressions describes be automatically caught and handled fairly quickly.

The CL-PCRE and Java regular expression engines don't have those somewhat expensive checks, and so are much more likely to encounter catastrophic behavior.