PatternSyntaxException

Why This Happens

PatternSyntaxException is thrown by Pattern.compile() and String methods that use regex (matches, replaceAll, split) when the regex pattern is malformed. Common issues include unescaped dots, brackets, or parentheses, and dangling metacharacters.

The Problem

String[] parts = "price: $10.00".split("$"); // $ is a regex metacharacter
String result = "hello (world)".replaceAll("(", ""); // Unclosed group

The Fix

String[] parts = "price: $10.00".split(Pattern.quote("$")); // Escape the pattern
String result = "hello (world)".replaceAll("\\(", ""); // Escaped parenthesis

Step-by-Step Fix

1. 1

   Identify the bad pattern

   Read the exception message for the pattern and the index where parsing failed.

2. 2

   Find unescaped metacharacters

   Check for unescaped regex special characters: . * + ? ^ $ { } [ ] ( ) | \

3. 3

   Escape or use Pattern.quote()

   Escape special characters with double backslash, or use Pattern.quote() to treat the entire string as literal.