FileCheck: strange regexp match behavior on CFI tests

I encountered CFI test failures in tests similar to compiler-rt/test/cfi/multiple-inheritance.cpp when using a libc implementation that outputs additional debug information.

The issue occurs with FileCheck patterns like:

```
// RUN: FileCheck -check-prefix=CFI -input-file %s %s
Some string 12
1

CFI: 1
CFI-NOT: {{^2$}}
```

This test fails, but if 1 is replaced with `{{^1$}}`, it passes.

Expected Behavior:

- `CFI: 1` should match the digit 1 on owns line
- `CFI-NOT: {{^2$}}` should not find 2 (because in this test there is no string that contains only '2')

Current Behavior:

- `CFI-NOT: {{^2$}}` pattern incorrectly matches, suggesting that after matching 1.

Questions:
- Is this the expected behavior of FileCheck?

Reproduce the command shown in compiler-rt/test/cfi/multiple-inheritance.cpp with the adjacent CFI and CFI-NOT patterns. Start by checking how FileCheck interprets line anchors and the search region after a positive match. Done means the behavior is explained and either corrected or documented as expected.

Written by the indexing model from the issue text.