Compute word boundary flags in start state.

At some point, I think I had convinced myself that we didn't need to
compute word boundary flags for the initial state, but it turns out that
we do.

Fixes #271

Author: BurntSushi

src/dfa.rs

@@ -1378,7 +1378,9 @@ impl<'a> Fsm<'a> {
              ((empty_flags.end as u8) << 1) |
              ((empty_flags.start_line as u8) << 2) |
              ((empty_flags.end_line as u8) << 3) |
-             ((state_flags.is_word() as u8) << 4))
+             ((empty_flags.word_boundary as u8) << 4) |
+             ((empty_flags.not_word_boundary as u8) << 5) |
+             ((state_flags.is_word() as u8) << 6))
             as usize
         };
         match self.cache.start_states[flagi] {
@@ -1412,9 +1414,17 @@ impl<'a> Fsm<'a> {
         empty_flags.end = text.len() == 0;
         empty_flags.start_line = at == 0 || text[at - 1] == b'\n';
         empty_flags.end_line = text.len() == 0;
-        if at > 0 && Byte::byte(text[at - 1]).is_ascii_word() {
+
+        let is_word_last = at > 0 && Byte::byte(text[at - 1]).is_ascii_word();
+        let is_word = at < text.len() && Byte::byte(text[at]).is_ascii_word();
+        if is_word_last {
             state_flags.set_word();
         }
+        if is_word == is_word_last {
+            empty_flags.not_word_boundary = true;
+        } else {
+            empty_flags.word_boundary = true;
+        }
         (empty_flags, state_flags)
     }
 
@@ -1433,9 +1443,18 @@ impl<'a> Fsm<'a> {
         empty_flags.end = text.len() == 0;
         empty_flags.start_line = at == text.len() || text[at] == b'\n';
         empty_flags.end_line = text.len() == 0;
-        if at < text.len() && Byte::byte(text[at]).is_ascii_word() {
+
+        let is_word_last =
+            at < text.len() && Byte::byte(text[at]).is_ascii_word();
+        let is_word = at > 0 && Byte::byte(text[at - 1]).is_ascii_word();
+        if is_word_last {
             state_flags.set_word();
         }
+        if is_word == is_word_last {
+            empty_flags.not_word_boundary = true;
+        } else {
+            empty_flags.word_boundary = true;
+        }
         (empty_flags, state_flags)
     }
 

tests/regression.rs

@@ -75,3 +75,10 @@ mat!(lits_unambiguous1, u!(r"(ABC|CDA|BC)X"), "CDAX", Some((0, 4)));
 // See: https://github.com/rust-lang-nursery/regex/issues/291
 mat!(lits_unambiguous2, u!(r"((IMG|CAM|MG|MB2)_|(DSCN|CIMG))(?P<n>[0-9]+)$"),
      "CIMG2341", Some((0, 8)), Some((0, 4)), None, Some((0, 4)), Some((4, 8)));
+
+// See: https://github.com/rust-lang-nursery/regex/issues/271
+mat!(end_not_wb, u!(r"$(?-u:\B)"), "\u{5c124}\u{b576c}", Some((8, 8)));
+mat!(endl_or_wb, u!(r"(?m:$)|(?-u:\b)"), "\u{6084e}", Some((4, 4)));
+mat!(zero_or_end, u!(r"(?i-u:\x00)|$"), "\u{e682f}", Some((4, 4)));
+mat!(y_or_endl, u!(r"(?i-u:y)|(?m:$)"), "\u{b4331}", Some((4, 4)));
+mat!(wb_start_x, u!(r"(?u:\b)^(?-u:X)"), "X", Some((0, 1)));