A regular expression search comes back: costly patterns refused, long lines windowed, 300 ms deadline

mvzr backtracks with no bound on its work (a*a*a*a*x over a hundred a's takes a
second, each repeat multiplying by the haystack length), and a search runs holding
the editor's turn, so one pasted pattern froze the editor. pardes does not write or
vendor a regex engine, so regexp.zig bounds what it hands mvzr: more than four
repeats is refused, a line longer than a window sized from measured worst cases is
searched in half-overlapping windows, and a deadline stops the search. addr names
each failure; normal s/S keeps what it found. The prescan also stops reading an
escaped backslash before n as a newline, and ends a class where mvzr does.

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

1 files changed, 4 insertions, 1 deletions

diff --git a/docs/helix-keys.md b/docs/helix-keys.md
index f2855b3a..087b83d0 100644
--- a/docs/helix-keys.md
+++ b/docs/helix-keys.md
@@ -249,7 +249,10 @@ compiles a runtime pattern with no allocator, called the way sam searches
 (`src/regexp.zig`, shared with a pane's `addr` file): each line is its own
 haystack, so `^` and `$` match at every line's start and end and `.` never
 crosses a newline, and a pattern naming `\n` runs over the whole selection.
-Where helix's `^` is only the selection's start, this is sam's.
+Where helix's `^` is only the selection's start, this is sam's. mvzr
+backtracks without bound, so the caps docs/fs.md gives for `addr` hold
+here too: more than four repeats will not compile (the selection is left
+alone) and a search past 300 ms keeps the matches it found so far.
 
 | Key | Behavior | Notes | Status |
 | --- | --- | --- | --- |