Fix a performance bug where using -w could result in very bad performance.

The specific issue is that -w causes the regex to be wrapped in Unicode
word boundaries. Regrettably, Unicode word boundaries are the one thing
our regex engine can't handle well in the presence of non-ASCII text. We
work around its slowness by stripping word boundaries in some
circumstances, and using the resulting expression as a way to produce match
candidates that are then verified by the full original regex.

This doesn't fix all cases, but it should fix all cases where -w is used.

grep/src/lib.rs

@@ -19,6 +19,7 @@ pub use search::{Grep, GrepBuilder, Iter, Match};
 mod literals;
 mod nonl;
 mod search;
+mod word_boundary;
 
 /// Result is a convenient type alias that fixes the type of the error to
 /// the `Error` type defined in this crate.

grep/src/search.rs

@@ -4,6 +4,8 @@ use syntax;
 
 use literals::LiteralSets;
 use nonl;
+use syntax::Expr;
+use word_boundary::strip_unicode_word_boundaries;
 use Result;
 
 /// A matched line.
@@ -127,22 +129,35 @@ impl GrepBuilder {
     pub fn build(self) -> Result<Grep> {
         let expr = try!(self.parse());
         let literals = LiteralSets::create(&expr);
-        let re = try!(
-            RegexBuilder::new(&expr.to_string())
-                .case_insensitive(self.opts.case_insensitive)
-                .multi_line(true)
-                .unicode(true)
-                .size_limit(self.opts.size_limit)
-                .dfa_size_limit(self.opts.dfa_size_limit)
-                .compile()
-        );
+        let re = try!(self.regex(&expr));
+        let required = literals.to_regex().or_else(|| {
+            let expr = match strip_unicode_word_boundaries(&expr) {
+                None => return None,
+                Some(expr) => expr,
+            };
+            debug!("Stripped Unicode word boundaries. New AST:\n{:?}", expr);
+            self.regex(&expr).ok()
+        });
         Ok(Grep {
             re: re,
-            required: literals.to_regex(),
+            required: required,
             opts: self.opts,
         })
     }
 
+    /// Creates a new regex from the given expression with the current
+    /// configuration.
+    fn regex(&self, expr: &Expr) -> Result<Regex> {
+        RegexBuilder::new(&expr.to_string())
+            .case_insensitive(self.opts.case_insensitive)
+            .multi_line(true)
+            .unicode(true)
+            .size_limit(self.opts.size_limit)
+            .dfa_size_limit(self.opts.dfa_size_limit)
+            .compile()
+            .map_err(From::from)
+    }
+
     /// Parses the underlying pattern and ensures the pattern can never match
     /// the line terminator.
     fn parse(&self) -> Result<syntax::Expr> {

grep/src/word_boundary.rs

@@ -0,0 +1,54 @@
+use syntax::Expr;
+
+/// Strips Unicode word boundaries from the given expression.
+///
+/// The key invariant this maintains is that the expression returned will match
+/// *at least* every where the expression given will match. Namely, a match of
+/// the returned expression can report false positives but it will never report
+/// false negatives.
+///
+/// If no word boundaries could be stripped, then None is returned.
+pub fn strip_unicode_word_boundaries(expr: &Expr) -> Option<Expr> {
+    // The real reason we do this is because Unicode word boundaries are the
+    // one thing that Rust's regex DFA engine can't handle. When it sees a
+    // Unicode word boundary among non-ASCII text, it falls back to one of the
+    // slower engines. We work around this limitation by attempting to use
+    // a regex to find candidate matches without a Unicode word boundary. We'll
+    // only then use the full (and slower) regex to confirm a candidate as a
+    // match or not during search.
+    use syntax::Expr::*;
+
+    match *expr {
+        Concat(ref es) if !es.is_empty() => {
+            let first = is_unicode_word_boundary(&es[0]);
+            let last = is_unicode_word_boundary(es.last().unwrap());
+            // Be careful not to strip word boundaries if there are no other
+            // expressions to match.
+            match (first, last) {
+                (true, false) if es.len() > 1 => {
+                    Some(Concat(es[1..].to_vec()))
+                }
+                (false, true) if es.len() > 1 => {
+                    Some(Concat(es[..es.len() - 1].to_vec()))
+                }
+                (true, true) if es.len() > 2 => {
+                    Some(Concat(es[1..es.len() - 1].to_vec()))
+                }
+                _ => None,
+            }
+        }
+        _ => None,
+    }
+}
+
+/// Returns true if the given expression is a Unicode word boundary.
+fn is_unicode_word_boundary(expr: &Expr) -> bool {
+    use syntax::Expr::*;
+
+    match *expr {
+        WordBoundary => true,
+        NotWordBoundary => true,
+        Group { ref e, .. } => is_unicode_word_boundary(e),
+        _ => false,
+    }
+}