Fix Regex::replacen when replacing an empty match.

Fixes rust-lang#393, rust-lang#394.

src/re_bytes.rs

@@ -494,19 +494,20 @@ impl Regex {
         mut rep: R,
     ) -> Cow<'t, [u8]> {
         if let Some(rep) = rep.no_expansion() {
+            let mut it = self.find_iter(text).enumerate().peekable();
+            if it.peek().is_none() {
+                return Cow::Borrowed(text);
+            }
             let mut new = Vec::with_capacity(text.len());
             let mut last_match = 0;
-            for (i, m) in self.find_iter(text).enumerate() {
+            for (i, m) in it {
                 if limit > 0 && i >= limit {
                     break
                 }
                 new.extend_from_slice(&text[last_match..m.start()]);
                 new.extend_from_slice(&rep);
                 last_match = m.end();
             }
-            if last_match == 0 {
-                return Cow::Borrowed(text);
-            }
             new.extend_from_slice(&text[last_match..]);
             return Cow::Owned(new);
         }

src/re_unicode.rs

@@ -583,19 +583,20 @@ impl Regex {
         //      replacements inside the replacement string. We just push it
         //      at each match and be done with it.
         if let Some(rep) = rep.no_expansion() {
+            let mut it = self.find_iter(text).enumerate().peekable();
+            if it.peek().is_none() {
+                return Cow::Borrowed(text);
+            }
             let mut new = String::with_capacity(text.len());
             let mut last_match = 0;
-            for (i, m) in self.find_iter(text).enumerate() {
+            for (i, m) in it {
                 if limit > 0 && i >= limit {
                     break
                 }
                 new.push_str(&text[last_match..m.start()]);
                 new.push_str(&rep);
                 last_match = m.end();
             }
-            if last_match == 0 {
-                return Cow::Borrowed(text);
-            }
             new.push_str(&text[last_match..]);
             return Cow::Owned(new);
         }

tests/replace.rs

@@ -36,3 +36,6 @@ replace!(no_expand2, replace,
 
 // See https://github.com/rust-lang/regex/issues/314
 replace!(match_at_start_replace_with_empty, replace_all, r"foo", "foobar", t!(""), "bar");
+
+// See https://github.com/rust-lang/regex/issues/393
+replace!(single_empty_match, replace, r"^", "bar", t!("foo"), "foobar");