fixed index out of bound when regex is empty

pyregexp/engine.py

@@ -212,7 +212,7 @@ def match_group(ast: Union[RE, GroupNode, OrNode], string: str) -> Tuple[bool, i
             '''
             nonlocal str_i
             backtrack_stack = []
-            curr_node = ast.children[0]
+            curr_node = ast.children[0] if len(ast.children) > 0 else None
             i = 0  # the children i'm iterating, not to confuse with str_i
 
             # the passed ast can't be a Leaf

setup.py

@@ -7,14 +7,14 @@
 setup(
     name='pyregexp',
     packages=['pyregexp'],
-    version='0.2.1',
+    version='0.2.2',
     license='MIT',
     description='Simple regex library',
     long_description=long_description,
     long_description_content_type='text/markdown',
     author='Lorenzo Felletti',
     url='https://github.com/lorenzofelletti/pyregex',
-    download_url='https://github.com/lorenzofelletti/pyregex/archive/v0.2.1.tar.gz',
+    download_url='https://github.com/lorenzofelletti/pyregex/archive/v0.2.2.tar.gz',
     keywords=['regex', 'regexp', 'engine'],
     install_requires=[],
     classifiers=[

test/test_engine.py

@@ -439,3 +439,24 @@ def test_ignore_case_casefolding(reng: RegexEngine):
     test_str = "acher"
     res, _ = reng.match(regex, test_str, ignore_case=2)
     assert res == False
+
+
+def test_empty_regex(reng: RegexEngine):
+    regex = r""
+    test_str = "aaaa"
+    res, _ = reng.match(regex, test_str)
+    assert res == True
+
+
+def test_empty_test_str(reng: RegexEngine):
+    regex = r"a"
+    test_str = ""
+    res, _ = reng.match(regex, test_str)
+    assert res == False
+
+
+def test_empty_regex_and_test_str(reng: RegexEngine):
+    regex = r""
+    test_str = ""
+    res, _ = reng.match(regex, test_str)
+    assert res == True