Reject an invalid regex when saving a match-and-replace rule

internal/rules/rules.go's apply() treats a regex that fails to compile
exactly the same as "no match" - it silently returns the text unchanged
with no error surfaced anywhere in the call chain. rules_save had no
validation before persisting, so a rule with a typo'd regex would save
successfully, show as Enabled in the UI, and simply never fire on any
traffic - no indication anything was wrong.

internal/ipc/server.go's "rules_save" handler now compiles r.Match with
regexp.Compile before persisting when IsRegex is set, rejecting with a
clear "invalid regex: ..." error otherwise. No TUI changes needed: the
existing ruleWriteDoneMsg error path already surfaces any saveRule
error inline via the status line and - since it only clears ruleForm on
success - keeps the form open with the user's draft intact so they can
fix the pattern without losing their edits. That path already existed
for other error classes (DB errors); this just adds a new one flowing
through it.

Verified live over the real protocol (raw JSON on the control socket):
an unclosed-bracket regex is rejected with the expected error and never
reaches the rules table; a valid regex rule still saves and returns
normally.

go build/vet/gofmt/test/mod tidy all clean.

1 files changed, 12 insertions, 0 deletions

diff --git a/internal/ipc/server.go b/internal/ipc/server.go
index 9602912..94073da 100644
--- a/internal/ipc/server.go
+++ b/internal/ipc/server.go
@@ -229,6 +229,18 @@ func (s *Server) handleConn(conn net.Conn) {
 				continue
 			}
 			r := *req.Rule
+			// A rule with a regex that fails to compile silently never
+			// matches anything at apply time (rules.apply treats a
+			// compile error the same as "no match") - the user would see
+			// it saved and enabled with no indication it's actually
+			// inert. Reject it here instead, at the point they can still
+			// fix it.
+			if r.IsRegex {
+				if _, err := regexp.Compile(r.Match); err != nil {
+					enc.Encode(Response{Type: "error", Error: "invalid regex: " + err.Error()})
+					continue
+				}
+			}
 			var err error
 			if r.ID == 0 {
 				r.ID, err = s.db.AddRule(r)