ISSUES-88: add check value of position in while cycle after increment

Author: sergeevik

src/main/java/com/dashjoin/jsonata/Tokenizer.java

@@ -139,7 +139,11 @@ Pattern scanRegex() {
                     throw new JException("S0301", position);
                 }
                 position++;
-                currentChar = path.charAt(position);
+                if (position < length) {
+                    currentChar = path.charAt(position);
+                } else {
+                    currentChar = 0;
+                }
                 // flags
                 start = position;
                 while (currentChar == 'i' || currentChar == 'm') {

src/test/java/com/dashjoin/jsonata/RegexTest.java

@@ -0,0 +1,22 @@
+package com.dashjoin.jsonata;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class RegexTest {
+    @Test
+    public void testRegex() {
+        var expression = Jsonata.jsonata("/^test.*$/");
+        Object evaluate = expression.evaluate(null);
+        String expected = "^test.*$";
+        Assertions.assertEquals(expected, evaluate.toString());
+    }
+
+    @Test
+    public void testEvalRegex() {
+        var expression = Jsonata.jsonata("$eval('/^test.*$/')");
+        Object evaluate = expression.evaluate(null);
+        String expected = "^test.*$";
+        Assertions.assertEquals(expected, evaluate.toString());
+    }
+}