Add examples (#10961)

core-java-modules/core-java-regex-2/src/test/java/com/baeldung/regex/lookaround/LookaroundUnitTest.java

@@ -0,0 +1,61 @@
+package com.baeldung.regex.lookaround;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.junit.jupiter.api.Test;
+
+public class LookaroundUnitTest {
+
+    @Test
+    void givenPositiveLookahead_thenSuccess() {
+        Pattern pattern = Pattern.compile("import (?=static.+).+");
+
+        Matcher matcher = pattern.matcher("import static org.junit.jupiter.api.Assertions.assertEquals;");
+        assertTrue(matcher.find());
+        assertEquals("import static org.junit.jupiter.api.Assertions.assertEquals;", matcher.group());
+
+        assertFalse(pattern.matcher("import java.util.regex.Matcher;")
+            .find());
+    }
+
+    @Test
+    void givenNegativeLookahead_thenSuccess() {
+        Pattern pattern = Pattern.compile("import (?!static.+).+");
+
+        Matcher matcher = pattern.matcher("import java.util.regex.Matcher;");
+        assertTrue(matcher.find());
+        assertEquals("import java.util.regex.Matcher;", matcher.group());
+
+        assertFalse(pattern.matcher("import static org.junit.jupiter.api.Assertions.assertEquals;")
+            .find());
+    }
+
+    @Test
+    void givenPositiveLookbehind_thenSuccess() {
+        Pattern pattern = Pattern.compile(".*(?<=jupiter).*assertEquals;");
+
+        Matcher matcher = pattern.matcher("import static org.junit.jupiter.api.Assertions.assertEquals;");
+        assertTrue(matcher.find());
+        assertEquals("import static org.junit.jupiter.api.Assertions.assertEquals;", matcher.group());
+
+        assertFalse(pattern.matcher("import static org.junit.Assert.assertEquals;")
+            .find());
+    }
+
+    @Test
+    void givenNegativeLookbehind_thenSuccess() {
+        Pattern pattern = Pattern.compile(".*(?<!jupiter.{0,30})assertEquals;");
+
+        Matcher matcher = pattern.matcher("import static org.junit.Assert.assertEquals;");
+        assertTrue(matcher.find());
+        assertEquals("import static org.junit.Assert.assertEquals;", matcher.group());
+
+        assertFalse(pattern.matcher("import static org.junit.jupiter.api.Assertions.assertEquals;")
+            .find());
+    }
+}