FULL PRODUCT VERSION :
java version "1.7.0_51"
Java(TM) SE Runtime Environment (build 1.7.0_51-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.51-b03, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.1.7601]

A DESCRIPTION OF THE PROBLEM :
"_".replaceAll("_", "\\") causes a StringIndexOutOfBoundsException.

This exception is not documented to be thrown from this method.
https://docs.oracle.com/javase/7/docs/api/java/lang/String.html#replaceAll(java.lang.String,%20java.lang.String)

I think the reason behind this is that the regex engine is seeing the escape character \, with nothing afterward to escape. In my opinion, this should instead be thrown as a PatternSyntaxException, as documented.

Additionally, "ABC".replaceAll("_", "\\") does not throw an exception, probably because the regex causing the exception is never evaluated. Perhaps this should fail fast, so that an invalid regex will not be a surprise during runtime.


REPRODUCIBILITY :
This bug can be reproduced always.