syntax: 'a{0}' should compile to Hir::empty

No matter what 'a' is, 'a{0}' is always equivalent to an empty regex.

Author: BurntSushi

regex-syntax/src/hir/mod.rs

@@ -311,6 +311,12 @@ impl Hir {
 
     /// Creates a repetition HIR expression.
     pub fn repetition(rep: Repetition) -> Hir {
+        // The regex 'a{0}' is always equivalent to the empty regex. This is
+        // true even when 'a' is an expression that never matches anything
+        // (like '\P{any}').
+        if rep.min == 0 && rep.max == Some(0) {
+            return Hir::empty();
+        }
         let mut info = HirInfo::new();
         info.set_always_utf8(rep.hir.is_always_utf8());
         info.set_all_assertions(rep.hir.is_all_assertions());

regex-syntax/src/hir/print.rs

@@ -372,6 +372,17 @@ mod tests {
         roundtrip("(?U)a{2}", "a{2}");
         roundtrip("(?U)a{1,}", "a+?");
         roundtrip("(?U)a{1,5}", "a{1,5}?");
+
+        // Test that various zero-length repetitions always translate to an
+        // empty regex. This is more a property of HIR's smart constructors
+        // than the printer though.
+        roundtrip("a{0}", "");
+        roundtrip("(?:ab){0}", "");
+        #[cfg(feature = "unicode-gencat")]
+        {
+            roundtrip(r"\p{any}{0}", "");
+            roundtrip(r"\P{any}{0}", "");
+        }
     }
 
     #[test]