BAEL-8025: How to fix PatternSyntaxException: Illegal repetition near… (#18654)

* BAEL-8025: How to fix PatternSyntaxException: Illegal repetition near index

* BAEL-8025: How to fix PatternSyntaxException: Illegal repetition near index

* BAEL-8025: How to fix PatternSyntaxException: Illegal repetition near index

Author: achraftt

core-java-modules/core-java-regex-4/src/test/java/com/baeldung/regex/exceptions/PatternSyntaxExceptionUnitTest.java

@@ -0,0 +1,65 @@
+package com.baeldung.regex.exceptions;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.regex.Pattern;
+import java.util.regex.PatternSyntaxException;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+class PatternSyntaxExceptionUnitTest {
+    // 3.1 Orphaned Quantifier
+    @Test
+    void givenOrphanedQuantifier_whenCompiled_thenThrowsPatternSyntaxException() {
+        assertThrows(PatternSyntaxException.class, () -> Pattern.compile("*[a-z]"));
+    }
+
+    @Test
+    void givenValidPatternForOrphanedQuantifierFix_whenCompiled_thenCompilesSuccessfully() {
+        Pattern.compile("[a-z]*abc"); // Fix: quantifier follows valid character class
+    }
+
+    // 3.2 Nested Quantifiers Without Grouping
+    @Test
+    void givenNestedQuantifiersWithoutGrouping_whenCompiled_thenThrowsPatternSyntaxException() {
+        assertThrows(PatternSyntaxException.class, () -> Pattern.compile("\\d+\\.?\\d+*"));
+    }
+
+    @Test
+    void givenGroupedNestedQuantifiers_whenCompiled_thenCompilesSuccessfully() {
+        Pattern.compile("\\d+(\\.\\d+)*"); // Fix: grouping allows stacking quantifiers
+    }
+
+    // 3.3 Unclosed or Malformed Curly Braces
+    @Test
+    void givenUnclosedCurlyBraces_whenCompiled_thenThrowsPatternSyntaxException() {
+        assertThrows(PatternSyntaxException.class, () -> Pattern.compile("\\d{2,"));
+    }
+
+    @Test
+    void givenValidCurlyBraces_whenCompiled_thenCompilesSuccessfully() {
+        Pattern.compile("\\d{2,4}"); // Fix: well-formed repetition syntax
+    }
+
+    // 3.4 Quantifying Unrepeatable or Improper Elements
+    @Test
+    void givenImproperQuantifierStacking_whenCompiled_thenThrowsPatternSyntaxException() {
+        assertThrows(PatternSyntaxException.class, () -> Pattern.compile("\\w+\\s+*"));
+    }
+
+    @Test
+    void givenProperlyGroupedQuantifier_whenCompiled_thenCompilesSuccessfully() {
+        Pattern.compile("(\\w+\\s+)*"); // Fix: quantifier applied to group
+    }
+
+    // 3.5 Escaping Literal Quantifier Characters
+    @Test
+    void givenUnescapedQuantifierCharacters_whenCompiled_thenThrowsPatternSyntaxException() {
+        assertThrows(PatternSyntaxException.class, () -> Pattern.compile("abc+*"));
+    }
+
+    @Test
+    void givenEscapedQuantifierCharacters_whenCompiled_thenCompilesSuccessfully() {
+        Pattern.compile("abc\\+\\*"); // Fix: escape special characters
+    }
+}