SyntaxWarnings emmitted when running `nbdev_export && nbdev_clean && nbdev_trust`

# Mminimally reproducible example

In the root of the repo run:

```sh
conda create -n nbdev1433 -c conda-forge nbdev jupyterlab
conda activate nbdev1433
nbdev_export && nbdev_clean && nbdev_trust
```

Expected output: None

Actual output:

```
❯ nbdev_export && nbdev_clean && nbdev_trust
:28: SyntaxWarning: invalid escape sequence '\F'
:28: SyntaxWarning: invalid escape sequence '\F'
:28: SyntaxWarning: invalid escape sequence '\F'
:28: SyntaxWarning: invalid escape sequence '\F'
:5: SyntaxWarning: invalid escape sequence '\|'
:5: SyntaxWarning: invalid escape sequence '\|'
:5: SyntaxWarning: invalid escape sequence '\|'
:5: SyntaxWarning: invalid escape sequence '\|'
:9: SyntaxWarning: invalid escape sequence '\.'
:7: SyntaxWarning: invalid escape sequence '\$'
:9: SyntaxWarning: invalid escape sequence '\.'
:7: SyntaxWarning: invalid escape sequence '\$'
:9: SyntaxWarning: invalid escape sequence '\.'
:7: SyntaxWarning: invalid escape sequence '\$'
:9: SyntaxWarning: invalid escape sequence '\.'
:7: SyntaxWarning: invalid escape sequence '\$'
:7: SyntaxWarning: invalid escape sequence '\s'
:7: SyntaxWarning: invalid escape sequence '\S'
:7: SyntaxWarning: invalid escape sequence '\s'
:7: SyntaxWarning: invalid escape sequence '\S'
:7: SyntaxWarning: invalid escape sequence '\s'
:7: SyntaxWarning: invalid escape sequence '\S'
:7: SyntaxWarning: invalid escape sequence '\s'
:7: SyntaxWarning: invalid escape sequence '\S'
:28: SyntaxWarning: invalid escape sequence '\F'
:9: SyntaxWarning: invalid escape sequence '\.'
:7: SyntaxWarning: invalid escape sequence '\$'
:5: SyntaxWarning: invalid escape sequence '\|'
:7: SyntaxWarning: invalid escape sequence '\s'
:7: SyntaxWarning: invalid escape sequence '\S'
```

Run the reproducible conda setup in the repository, then trace the nbdev_export, nbdev_clean, and nbdev_trust entry points to find where notebook code produces the SyntaxWarnings. Done means the command sequence completes without emitting those warnings, while its expected behavior remains unchanged.

Written by the indexing model from the issue text.