A DESCRIPTION OF THE PROBLEM :
Compiling a Regex Pattern with an unescaped trailing backslash causes an internal error:
Pattern.compile("\\")

Exception:
Exception java.util.regex.PatternSyntaxException: Unexpected internal error near index 1

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This report is about the fact that an internal error is reported; it is not (like JDK-4783892) about the misunderstanding that backslashes need to be escaped.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Pattern.compile("\\")

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
An exception with a message saying something similar to:
> Unescaped trailing backslash at index ...
ACTUAL -
Exception java.util.regex.PatternSyntaxException: Unexpected internal error near index 1