Adding "Filtering List using regex code" | BAEL-8736 (#18098)

* Adding filtering using regex code

* moving files to different package

* correcting indents

* organizing files

* Update README.md

* Update README.md

* updating message name

* updated test class

Author: programenth

core-java-modules/core-java-regex-3/src/main/java/com/baeldung/listregexfilter/RegexFilterExample.java

@@ -0,0 +1,61 @@
+package com.baeldung.listregexfilter;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import java.util.stream.Collectors;
+
+public class RegexFilterExample {
+
+    public List<String> filterUsingPatternAndPredicate() {
+        List<String> fruits = List.of("apple", "banana", "cherry", "apricot", "avocado");
+
+        Pattern pattern = Pattern.compile("^a.*");
+
+        return fruits.stream()
+          .filter(pattern.asPredicate())
+          .toList();
+    }
+
+    public List<String> filterUsingStringMatches() {
+        List<String> list = List.of("123", "abc", "456def", "789", "xyz");
+
+        return list.stream()
+          .filter(str -> str.matches("\\d+"))
+          .toList();
+    }
+
+    public List<String> filterUsingPatternCompile() {
+        List<String> numbers = List.of("one", "two", "three", "four", "five");
+        List<String> startWithTList = new ArrayList<>();
+
+        Pattern pattern = Pattern.compile("^t.*");
+
+        for (String item : numbers) {
+            Matcher matcher = pattern.matcher(item);
+            if (matcher.matches())
+                startWithTList.add(item);
+        }
+
+        return startWithTList;
+    }
+
+    public Map<Boolean, List<String>> filterUsingCollectorsPartitioningBy() {
+        List<String> fruits = List.of("apple", "banana", "apricot", "berry");
+
+        Pattern pattern = Pattern.compile("^a.*");
+
+        return fruits.stream()
+          .collect(Collectors.partitioningBy(pattern.asPredicate()));
+    }
+
+    public static void main(String[] args) {
+        RegexFilterExample regexFilterExample = new RegexFilterExample();
+        regexFilterExample.filterUsingPatternAndPredicate();
+        regexFilterExample.filterUsingStringMatches();
+        regexFilterExample.filterUsingPatternCompile();
+        regexFilterExample.filterUsingCollectorsPartitioningBy();
+    }
+}

core-java-modules/core-java-regex-3/src/test/java/com/baeldung/listregexfilter/RegexFilterExampleUnitTest.java

@@ -0,0 +1,37 @@
+package com.baeldung.listregexfilter;
+
+import java.util.List;
+import java.util.Map;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+class RegexFilterExampleUnitTest {
+
+    RegexFilterExample regexFilterExample = new RegexFilterExample();
+
+    @Test
+    void whenfilterUsingPatternAndPredicateCalled_thenReturnElementsStartingWithA() {
+        List<String> newList = regexFilterExample.filterUsingPatternAndPredicate();
+        Assertions.assertEquals(List.of("apple", "apricot", "avocado"), newList);
+    }
+
+    @Test
+    void whenfilterUsingStringMatchesCalled_thenReturnElementsContainingDig() {
+        List<String> newList = regexFilterExample.filterUsingStringMatches();
+        Assertions.assertEquals(List.of("123", "789"), newList);
+    }
+
+    @Test
+    void whenfilterUsingPatternCompileCalled_thenReturnElementsStartingWithT3() {
+        List<String> newList = regexFilterExample.filterUsingPatternCompile();
+        Assertions.assertEquals(List.of("two", "three"), newList);
+    }
+
+    @Test
+    void whenfilterUsingCollectorsPartitioningByCalled_thenReturnElementsStartingWithA4() {
+        Map<Boolean, List<String>> newList = regexFilterExample.filterUsingCollectorsPartitioningBy();
+        Assertions.assertEquals(List.of("apple", "apricot"), newList.get(true));
+        Assertions.assertEquals(List.of("banana", "berry"), newList.get(false));
+    }
+}