[regexp] Fix numbered reference before named capture

test/mjsunit/harmony/regexp-named-captures.js

Index: test/mjsunit/harmony/regexp-named-captures.js
diff --git a/test/mjsunit/harmony/regexp-named-captures.js b/test/mjsunit/harmony/regexp-named-captures.js
index e4de7b00c8db9ea7f63d14f8fb4c5dc86e8b220c..67d62728eff82a91e88f76366121f3d4dce78d51 100644
--- a/test/mjsunit/harmony/regexp-named-captures.js
+++ b/test/mjsunit/harmony/regexp-named-captures.js
@@ -97,6 +97,12 @@ assertThrows('/(?<𐒤>a)/u', SyntaxError);  // ID_Continue but not ID_Start.
 assertEquals(undefined, /(?<a>.)/u.exec("a").groups.__proto__);
 assertEquals("a", /(?<__proto__>a)/u.exec("a").groups.__proto__);
 
+// Backreference before the group (exercises the capture mini-parser).
+assertThrows("/\\1(?:.)/u", SyntaxError);
+assertThrows("/\\1(?<=a)./u", SyntaxError);
+assertThrows("/\\1(?<!a)./u", SyntaxError);
+assertEquals(["a", "a"], /\1(?<a>.)/u.exec("abcd"));
+
 // @@replace with a callable replacement argument (no named captures).
 {
   let result = "abcd".replace(/(.)(.)/u, (match, fst, snd, offset, str) => {