Resolving PatternSyntaxException: Unclosed character class

PatternSyntaxException with 'Unclosed character class' is a common Java regex error caused by missing closing brackets or unescaped special characters. The post explains how to reproduce the error, fix it by properly closing character classes, and covers common causes like missing brackets and incorrect nested patterns. A practical example shows how String.split() triggers this exception when square brackets in the delimiter aren't escaped, and demonstrates the correct fix using double-escaped brackets (\], \[).