When do I get this error?

The "Unclosed regular expression" error is thrown when JSLint or JSHint encounters a regular expression literal with no closing / character. Here's an example:

var regex = /^unclosed$;

Why do I get this error?

This error is raised to highlight a fatal JavaScript syntax error. Your code will not run unless you fix this issue. The ECMAScript 5 specification lists the following grammar for regular expression literals (ES5 §7.8.5):

RegularExpressionLiteral ::
    / RegularExpressionBody / RegularExpressionFlags

This production makes it clear that regular expression literal bodies must be terminated by a / character. Not doing so will always cause a syntax error. To fix this issue, simply close the regular expression in question:

var regex = /^closed$/;

In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax. Since this message relates to a fatal syntax error you cannot disable it.