replace does not work against regex '^'.

[fulara]

I was just trying different corner cases with rust regex and I came across this one:
Code:

extern crate regex;
use regex::Regex;
fn main() {
    let regex = Regex::new("^").unwrap();
    let s = regex.replace("_", "begin").into_owned();
    assert_eq!("begin_", s); // will fail, string still '_'
}

Above fails. and below succeeds:

extern crate regex;
use regex::Regex;
fn main() {
    let regex = Regex::new("$").unwrap();
    let s = regex.replace("_", "begin").into_owned();
    assert_eq!("_begin", s); // will pass.
}

Tested against 0.2.2.

I tried testing the same case with boost regex and sed and both of these yielded expected result.