Fix regex evaluation

Fixes #13

Author: rayokota

src/jsonata/tokenizer.py

@@ -138,7 +138,10 @@ def scan_regex(self) -> re.Pattern:
                 start = self.position
                 while current_char == 'i' or current_char == 'm':
                     self.position += 1
-                    current_char = self.path[self.position]
+                    if self.position < self.length:
+                        current_char = self.path[self.position]
+                    else:
+                        current_char = None
                 flags = self.path[start:self.position] + 'g'
 
                 # Convert flags to Java Pattern flags

tests/string_test.py

@@ -38,6 +38,10 @@ def test_replace(self):
         assert jsonata.Jsonata("$replace('h.ello', '.', '')").evaluate(None) == "hello"
         assert jsonata.Jsonata("$replace('h.e.l.l.o', '.', '',2)").evaluate(None) == "hel.l.o"
 
+    def test_regex(self):
+        assert (jsonata.Jsonata("($matcher := $eval('/^' & 'foo' & '/i'); $.$spread()[$.$keys() ~> $matcher])")
+                .evaluate({"foo": 1, "bar": 2}) == {"foo": 1})
+
     #
     # Additional $split tests
     #