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'
```

Esegui la configurazione conda riproducibile nel repository, quindi segui gli entry point nbdev_export, nbdev_clean e nbdev_trust per trovare dove il codice del notebook produce le SyntaxWarnings. Il lavoro è completato quando la sequenza di comandi termina senza emettere questi avvisi, mantenendo invariato il comportamento previsto.

Scritto dal modello di indicizzazione a partire dal testo della issue.