A pattern anchoring some alternatives with ^ and not others is refused with why, not a bare bad regular expression

`^def|x` and `foo$|^zèbre` were refused with a bare "bad regular expression",
the same words as a syntax error, so a script could not tell what to change.
They now say "an alternation anchors every branch with ^ or none (^a|^b, not
^a|b)", through addr and Edit alike. fs.md states the rule and that a `$`
does not count toward it (`foo$|bar` is fine).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/ninep/addr.zig b/src/ninep/addr.zig
index 33a3f646..9674a323 100644
--- a/src/ninep/addr.zig
+++ b/src/ninep/addr.zig
@@ -304,6 +304,7 @@ pub const Addr = struct {
                 error.TooLong => regexp_.Regex.e_long,
                 error.WideRange => regexp_.Regex.e_wide,
                 error.NegatedRunes => regexp_.Regex.e_negated,
+                error.Mixed => regexp_.Regex.e_mixed,
                 error.Bad => e_regexp,
             };
             return null;
@@ -383,6 +384,18 @@ test "an address is evaluated from the current one, as acme's are" {
     try testing.expectEqualStrings(e_order, th.wr(q, qaddr, "/a/,/b/").reply.ename);
 }
 
+test "an alternation anchoring some branches and not others is refused with why, a $ not counting" {
+    const p = try th.withFile(testing.allocator, "def\nfoo\n");
+    defer p.deinit();
+    const node = Node.of(th.serialOf(p), .addr);
+    for ([_][]const u8{ "/^def|x/", "/foo$|^zèbre/" }) |pat| {
+        const r = th.wr(p, node, pat);
+        try testing.expectEqual(E.INVAL, r.errno());
+        try testing.expectEqualStrings(regexp_.Regex.e_mixed, r.reply.ename);
+    }
+    try testing.expectEqual(Status.ok, th.wr(p, node, "/foo$|def/").reply.status);
+}
+
 test "a pattern with a newline and an anchor mvzr would misread is refused EINVAL, by addr and by Edit" {
     const p = try th.withFile(testing.allocator, "a\nfoo\n");
     defer p.deinit();