coregex.MustCompile(^x(a|bc)).MatchString("xbc") // false #166

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1. kolkov commented on Oct 8, 2026

   

   kolkov

   on Oct 8, 2026

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   @vmihailenco Thanks for the report, and apologies for the six-week silence — I somehow missed the notification and only caught this when reviewing the repo today. In parallel with coregex I've been developing a headless IDE (open-sourcing soon) for managing large Go codebases like GoGPU via MCP — think console GoLand with AI automation — along with daemon, fursy and relica.

   Confirmed and root-caused. ^x(a|bc) is the tip of two bugs in the anchored fast paths. Both produce wrong answers on common pattern shapes (routers, HTTP version checks, SemVer). The submatch path is not affected; Match, Find, FindAll, Count and ReplaceAll are.

   1. BranchDispatch ignores everything around the alternation. IsBranchDispatchPattern accepts any anchored concat that contains an alternation, but the dispatcher assumes the alternation starts at position 0 and ends the pattern:

   ^x(a|bc)              "xbc"        → false   (your report)
   ^HTTP/(1\.0|1\.1|2)   "HTTP/2"     → false
   ^/api/(users|posts)   "/api/users" → false
   ^(GET|POST) /         "GET x"      → true    (false positive: suffix ignored)
   ^(a\d|b)              "a"          → true    (false positive: non-literal branch "assumed true")
   

   2. First-byte extraction ignores FoldCase. regexp/syntax rewrites [Vv] as (?i:V); the extractor takes only the uppercase rune, so the fast-reject path discards lowercase input:

   ^[Vv]\d+   "v1" → false   ("V1" → true)
   

   Fix is in progress; I'll link the PR here and cut a patch release as soon as it lands. It will also add a cross-API invariant test for anchored patterns — MatchString(s) == (FindStringIndex(s) != nil) == (FindStringSubmatchIndex(s) != nil) — which is exactly the signal this class of bug produces.

   👍React with 👍1Reacted by Vladimir Mihailenco
2. kolkov

   added

   type: bugSomething isn't workingSomething isn't working

   priority: criticalRelease blocker, correctness regressionRelease blocker, correctness regression

   area: metaStrategy selection, engine orchestrationStrategy selection, engine orchestration

   area: nfaThompson NFA, PikeVM, BoundedBacktrackerThompson NFA, PikeVM, BoundedBacktracker

   on Oct 8, 2026
3. kolkov

   closed this as completedin #172on Oct 9, 2026
4. kolkov

   self-assigned this

   on Oct 9, 2026
5. kolkov commented on Oct 9, 2026

   

   kolkov

   on Oct 9, 2026

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   Fixed in v0.12.27 — PR #172.