regex search: `|` alternation silently returns 0 matches (false negatives)

[justrach]

Summary

In search / faster_search with mode=regex (flavor zigrep_basic), the | alternation operator does not work and the search returns 0 matches silently — no error, no warning. This reads as a confident "pattern not found" when the terms are actually present, which is actively misleading and caused me to draw wrong conclusions while searching a codebase.

Reproduction

File index.ts line 1:

import { generateText, streamText, createGateway } from "ai";

pattern	mode	result	expected
createGateway	regex	2 matches ✓	2
generateText|streamText	regex	0 matches ✗	matches line 1
nonexistentword|createGateway	regex	0 matches ✗	matches createGateway

The single-term regex finds it; the moment an alternation | is added, the whole query returns 0 — even though one of the alternatives clearly matches.

Impact

• Silent false negatives. A multi-term a|b|c "does any of these exist" sweep — a very common first-pass search — returns 0 and looks authoritative.
• No signal that alternation is unsupported. If zigrep_basic is intentionally POSIX BRE (where | is a literal char, alternation requires \|), that's a reasonable engine choice, but it's a footgun without feedback.

Suggested fix (any one)

1. Support ERE-style | alternation in mode=regex, or
2. If | is literal in BRE, detect a bare | in a regex pattern and surface a hint (e.g. "| is literal in basic regex; use \\| for alternation or split into multiple queries"), or
3. At minimum, document the flavor's alternation semantics in the tool description.

Environment

• codedbpro v0.2.9
• Both search and faster_search regex modes affected (same backend).

[justrach]

Fixed in 31233f9 (decomposeRegex falls back to scan-all when any alternation branch yields no trigrams). Merged to release/0.2.5825 (tip 503623f) and release/0.2.5826; also in PR #630 → main. Regression test: issue-628: alternation with a no-trigram branch falls back to scan-all in src/test_index.zig — verified to fail pre-fix (expected 0, found 1).

Note: this fixes codedb's own regex engine (index.zig). If codedbpro's mode=regex uses a separate zigrep_basic backend, that may need its own check.