Add Captures::get example.

The example shows how to get the matched text of any capture group while
defaulting to the empty string if that particular group didn't participate
in the match.

Fixes #338

Author: BurntSushi

Diff for: src/re_bytes.rs

@@ -789,6 +789,22 @@ impl<'t> Captures<'t> {
     /// Returns the match associated with the capture group at index `i`. If
     /// `i` does not correspond to a capture group, or if the capture group
     /// did not participate in the match, then `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// Get the text of the match with a default of an empty string if this
+    /// group didn't participate in the match:
+    ///
+    /// ```rust
+    /// # use regex::bytes::Regex;
+    /// let re = Regex::new(r"[a-z]+(?:([0-9]+)|([A-Z]+))").unwrap();
+    /// let caps = re.captures(b"abc123").unwrap();
+    ///
+    /// let text1 = caps.get(1).map_or(&b""[..], |m| m.as_bytes());
+    /// let text2 = caps.get(2).map_or(&b""[..], |m| m.as_bytes());
+    /// assert_eq!(text1, &b"123"[..]);
+    /// assert_eq!(text2, &b""[..]);
+    /// ```
     pub fn get(&self, i: usize) -> Option<Match<'t>> {
         self.locs.pos(i).map(|(s, e)| Match::new(self.text, s, e))
     }

Diff for: src/re_unicode.rs

@@ -934,6 +934,22 @@ impl<'t> Captures<'t> {
     /// Returns the match associated with the capture group at index `i`. If
     /// `i` does not correspond to a capture group, or if the capture group
     /// did not participate in the match, then `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// Get the text of the match with a default of an empty string if this
+    /// group didn't participate in the match:
+    ///
+    /// ```rust
+    /// # use regex::Regex;
+    /// let re = Regex::new(r"[a-z]+(?:([0-9]+)|([A-Z]+))").unwrap();
+    /// let caps = re.captures("abc123").unwrap();
+    ///
+    /// let text1 = caps.get(1).map_or("", |m| m.as_str());
+    /// let text2 = caps.get(2).map_or("", |m| m.as_str());
+    /// assert_eq!(text1, "123");
+    /// assert_eq!(text2, "");
+    /// ```
     pub fn get(&self, i: usize) -> Option<Match<'t>> {
         self.locs.pos(i).map(|(s, e)| Match::new(self.text, s, e))
     }