bug: fix CaptureLocations::get to handle invalid offsets

The contract of this function says that any invalid group offset should
result in a return value of None. In general, it worked fine, unless the
offset was so big that some internal multiplication overflowed. That
could in turn produce an incorrect result or a panic. So we fix that
here with checked arithmetic.

Fixes #738, Fixes #950

src/re_bytes.rs

@@ -896,6 +896,27 @@ impl<'r> FusedIterator for CaptureNames<'r> {}
 /// In order to build a value of this type, you'll need to call the
 /// `capture_locations` method on the `Regex` being used to execute the search.
 /// The value returned can then be reused in subsequent searches.
+///
+/// # Example
+///
+/// This example shows how to create and use `CaptureLocations` in a search.
+///
+/// ```
+/// use regex::bytes::Regex;
+///
+/// let re = Regex::new(r"(?<first>\w+)\s+(?<last>\w+)").unwrap();
+/// let mut locs = re.capture_locations();
+/// let m = re.captures_read(&mut locs, b"Bruce Springsteen").unwrap();
+/// assert_eq!(0..17, m.range());
+/// assert_eq!(Some((0, 17)), locs.get(0));
+/// assert_eq!(Some((0, 5)), locs.get(1));
+/// assert_eq!(Some((6, 17)), locs.get(2));
+///
+/// // Asking for an invalid capture group always returns None.
+/// assert_eq!(None, locs.get(3));
+/// assert_eq!(None, locs.get(34973498648));
+/// assert_eq!(None, locs.get(9944060567225171988));
+/// ```
 #[derive(Clone, Debug)]
 pub struct CaptureLocations(re_trait::Locations);
 

src/re_trait.rs

@@ -20,7 +20,7 @@ impl Locations {
     /// not match anything. The positions returned are *always* byte indices
     /// with respect to the original string matched.
     pub fn pos(&self, i: usize) -> Option<(usize, usize)> {
-        let (s, e) = (i * 2, i * 2 + 1);
+        let (s, e) = (i.checked_mul(2)?, i.checked_mul(2)?.checked_add(1)?);
         match (self.0.get(s), self.0.get(e)) {
             (Some(&Some(s)), Some(&Some(e))) => Some((s, e)),
             _ => None,

src/re_unicode.rs

@@ -906,6 +906,27 @@ impl<'r, 't> FusedIterator for SplitN<'r, 't> {}
 /// In order to build a value of this type, you'll need to call the
 /// `capture_locations` method on the `Regex` being used to execute the search.
 /// The value returned can then be reused in subsequent searches.
+///
+/// # Example
+///
+/// This example shows how to create and use `CaptureLocations` in a search.
+///
+/// ```
+/// use regex::Regex;
+///
+/// let re = Regex::new(r"(?<first>\w+)\s+(?<last>\w+)").unwrap();
+/// let mut locs = re.capture_locations();
+/// let m = re.captures_read(&mut locs, "Bruce Springsteen").unwrap();
+/// assert_eq!(0..17, m.range());
+/// assert_eq!(Some((0, 17)), locs.get(0));
+/// assert_eq!(Some((0, 5)), locs.get(1));
+/// assert_eq!(Some((6, 17)), locs.get(2));
+///
+/// // Asking for an invalid capture group always returns None.
+/// assert_eq!(None, locs.get(3));
+/// assert_eq!(None, locs.get(34973498648));
+/// assert_eq!(None, locs.get(9944060567225171988));
+/// ```
 #[derive(Clone, Debug)]
 pub struct CaptureLocations(re_trait::Locations);