不能在Rust文档测试中使用依赖包

我正在尝试为Rust写的一个项目编写文档.其中一个文档需要使用regex::Regex.这是我要写的文档:

/// Return a list of the offsets of the tokens in `s`, as a sequence of `(start, end)`
/// tuples, by splitting the string at each successive match of `regex`.
///
/// # Examples
///
/// ```
/// use rusty_nltk::tokenize::util::regexp_span_tokenize;
/// use regex::Regex;
///
/// let s = "Good muffins cost $3.88\nin New York.  Please buy me
///         two of them.\n\nThanks.";
/// let regex = regex::Regex::new(r"\s").unwrap();
/// let spans = regexp_span_tokenize(s, regex).unwrap();
/// ```

它给了我这个错误:

---- tokenize::util::regexp_span_tokenize_0 stdout ----
    :4:9: 4:14 error: unresolved import `regex::Regex`. Maybe a missing `extern crate regex`? [E0432]
:4     use regex::Regex;
                 ^~~~~
error: aborting due to previous error

---- tokenize::util::regexp_span_tokenize_0 stdout ----
    :3:9: 3:19 error: unresolved import `rusty_nltk::tokenize::util::regexp_span_tokenize`. Maybe a missing `extern crate rusty_nltk`? [E0432]
:3     use rusty_nltk::tokenize::util::regexp_span_tokenize;
                 ^~~~~~~~~~
:4:9: 4:14 error: unresolved import `regex::Regex`. Did you mean `self::regex`? [E0432]
:4     use regex::Regex;
                 ^~~~~
error: aborting due to 2 previous errors

extern crate regex;
pub mod tokenize;

extern crate regex;
pub mod util;

extern crate regex; 
use regex::Regex;