Lower backtrack_limit to fail earlier for invalid input

src/lib.rs

@@ -6,6 +6,7 @@ use std::num::NonZeroU64;
 use std::thread;
 
 use fancy_regex::Regex;
+use fancy_regex::RegexBuilder;
 use pyo3::exceptions;
 use pyo3::prelude::*;
 use pyo3::pyclass;
@@ -417,7 +418,7 @@ impl CoreBPE {
         special_tokens_encoder: HashMap<String, Rank>,
         pattern: &str,
     ) -> PyResult<Self> {
-        let regex = Regex::new(pattern)
+        let regex = RegexBuilder::new(pattern).backtrack_limit(100_000).build()
             .map_err(|e| PyErr::new::<exceptions::PyValueError, _>(e.to_string()))?;
 
         let special_regex = {
@@ -572,6 +573,7 @@ fn _tiktoken(_py: Python, m: &PyModule) -> PyResult<()> {
 
 #[cfg(test)]
 mod tests {
+    use fancy_regex::RegexBuilder;
     use rustc_hash::FxHashMap as HashMap;
 
     use crate::{byte_pair_split, Rank};
@@ -596,4 +598,16 @@ mod tests {
         let res = byte_pair_split(b"abab", &ranks);
         assert_eq!(res, vec![b"ab", b"ab"]);
     }
+
+    #[test]
+    fn test_effect_of_backtrack_limit() {
+        let regex = RegexBuilder::new(r"(a|b|ab)*(?=c)")
+            .backtrack_limit(10)
+            .build()
+            .expect("Failed to build regex")
+            .clone();
+
+        let input = "ab".repeat(100) + "c";
+        assert!(regex.is_match(&input).is_err(), "Should throw");
+    }
 }

tests/test_encoding.py

@@ -11,14 +11,13 @@
 from .test_helpers import ENCODING_FACTORIES, MAX_EXAMPLES
 
 
-@pytest.mark.skip(reason="Takes a really long time to finish, but was added to reproduce a crash.")
 @pytest.mark.parametrize("make_enc", ENCODING_FACTORIES)
 def test_extremely_big_encoding(make_enc: Callable[[], tiktoken.Encoding]):
     enc = make_enc()
     for c in ["^", "0", "a", "'s"]: # TODO " ", "\n" are still failing
         print(f"Validating `{c}`")
 
-        big_value = c * 1_000_000
+        big_value = c * 100_000
         assert big_value == enc.decode(enc.encode(big_value))
 
         big_value = " " + big_value