Update README.md

java-strings/src/test/java/com/baeldung/stringisnumeric/RegularExpressionsUnitTest.java

@@ -0,0 +1,22 @@
+package com.baeldung.stringisnumeric;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.Test;
+
+public class RegularExpressionsUnitTest {
+    public static boolean isNumeric(String strNum) {
+        return strNum.matches("-?\\d+(\\.\\d+)?");
+    }
+    
+    @Test
+    public void whenUsingRegularExpressions_thenTrue() {
+        // Valid Numbers
+        assertThat(isNumeric("22")).isTrue();
+        assertThat(isNumeric("5.05")).isTrue();
+        assertThat(isNumeric("-200")).isTrue();
+         
+        // Invalid Numbers
+        assertThat(isNumeric("abc")).isFalse();
+    }
+}