sec 13.3 (regexp): raw strings code is not shown  

In https://learningds.org/ch/13/text_regex.html you write `Note that we add the r character before the quotes to create a raw string, which makes regexes easier to write.` but the corresponding source code is not shown, so this statement is unclear.
```
show_regex_match(r'\d\d\d-\d\d-\d\d\d\d', 'My other number is 6382-13-38420.')
```