search: zero-width regex found repeatedly

This issue is best described by example (MWE):
- open/create any notebook. It can even be completely empty
- search with enabled regex: `^` (`$` works the same)

result:
---

The same occurrence is found over and over again. A warning is displayed as well:
```Warning: too many matches (101). Some changes might not be shown or applied.```
the count may differ from 101.

expected result:
---

Every beginning of line is found once.

Reproduce the issue in an empty notebook by enabling regex search and searching for `^`, then compare with `$`. Trace the notebook search entry point and make the result show each line start once without the repeated-match warning; verify the same behavior for both expressions.

Written by the indexing model from the issue text.