syntax: fix counted repetition bug

This fixes a bug where the HIR translator would panic on regexes such as
`(?i){1}` since it assumes that every repetition operator has a valid
sub-expression, and `(?i)` is not actually a sub-expression (but is more
like a directive instead).

Previously, we fixed this same bug for *uncounted* repetitions in commit
17764ff (for bug #465), but we did not fix it for counted repetitions.
We apply the same fix here.

Fixes #555

Author: BurntSushi

CHANGELOG.md

@@ -10,6 +10,8 @@ Performance improvements:
 
 Bug fixes:
 
+* [BUG #555](https://github.com/rust-lang/regex/issues/555):
+  Fix a bug where the parser would panic on patterns like `(?m){1,1}`.
 * [BUG #557](https://github.com/rust-lang/regex/issues/557):
   Fix a bug where captures could lead to an incorrect match.
 

regex-syntax/src/ast/parse.rs

@@ -1100,6 +1100,13 @@ impl<'s, P: Borrow<Parser>> ParserI<'s, P> {
                 ast::ErrorKind::RepetitionMissing,
             )),
         };
+        match ast {
+            Ast::Empty(_) | Ast::Flags(_) => return Err(self.error(
+                self.span(),
+                ast::ErrorKind::RepetitionMissing,
+            )),
+            _ => {}
+        }
         if !self.bump_and_bump_space() {
             return Err(self.error(
                 Span::new(start, self.pos()),
@@ -3124,6 +3131,18 @@ bar
                 ast: Box::new(lit('a', 0)),
             })));
 
+        assert_eq!(
+            parser(r"(?i){0}").parse().unwrap_err(),
+            TestError {
+                span: span(4..4),
+                kind: ast::ErrorKind::RepetitionMissing,
+            });
+        assert_eq!(
+            parser(r"(?m){1,1}").parse().unwrap_err(),
+            TestError {
+                span: span(4..4),
+                kind: ast::ErrorKind::RepetitionMissing,
+            });
         assert_eq!(
             parser(r"a{").parse().unwrap_err(),
             TestError {

tests/regression.rs

@@ -14,6 +14,12 @@ fn regression_many_repeat_stack_overflow() {
     assert_eq!(vec![(0, 1)], findall!(re, "a"));
 }
 
+// See: https://github.com/rust-lang/regex/issues/555
+#[test]
+fn regression_invalid_repetition_expr() {
+    assert!(regex_new!("(?m){1,1}").is_err());
+}
+
 // See: https://github.com/rust-lang/regex/issues/75
 mat!(regression_unsorted_binary_search_1, r"(?i)[a_]+", "A_", Some((0, 2)));
 mat!(regression_unsorted_binary_search_2, r"(?i)[A_]+", "a_", Some((0, 2)));