Fix oneOf implementation of the json schema spec

Implements XOR regex using negative lookaheads.

outlines/text/json_schema.py

@@ -137,7 +137,16 @@ def to_regex(resolver: Resolver, instance: dict):
     # one of the given subschemas.
     elif "oneOf" in instance:
         subregexes = [to_regex(resolver, t) for t in instance["oneOf"]]
-        return rf"({'|'.join(subregexes)})"
+
+        xor_patterns = []
+        # json schema validation ensured there is no overlapping schemas in oneOf
+        for subregex in subregexes:
+            other_subregexes = filter(lambda r: r != subregex, subregexes)
+            other_subregexes_str = "|".join([f"{s}" for s in other_subregexes])
+            negative_lookahead = f"(?!.*({other_subregexes_str}))"
+            xor_patterns.append(f"({subregex}){negative_lookahead}")
+
+        return rf"({'|'.join(xor_patterns)})"
 
     # The enum keyword is used to restrict a value to a fixed set of values. It
     # must be an array with at least one element, where each element is unique.

tests/text/test_json_schema.py

@@ -220,10 +220,19 @@ def test_match_number(pattern, does_match):
         (
             {
                 "title": "Foo",
-                "oneOf": [{"type": "string"}, {"type": "number"}],
+                "oneOf": [{"type": "string"}, {"type": "number"}, {"type": "boolean"}],
             },
-            rf"({STRING}|{NUMBER})",
-            [("12.3", True), ('"a"', True), ('1.3"a"', False)],
+            rf"(({STRING})(?!.*({NUMBER}|{BOOLEAN}))|({NUMBER})(?!.*({STRING}|{BOOLEAN}))|({BOOLEAN})(?!.*({STRING}|{NUMBER})))",
+            [
+                ("12.3", True),
+                ("true", True),
+                ('"a"', True),
+                ("null", False),
+                ("", False),
+                ("12true", False),
+                ('1.3"a"', False),
+                ('12.3true"a"', False),
+            ],
         ),
         # anyOf
         (