python regex optional capture group

Question

I have the following problem matching the needed data from filenames like this: ``` miniseries.season 1.part 5.720p.avi miniseries.part 5.720p.avi miniseries.part VII.720p.avi # episode or season expressed in Roman numerals ``` The "season XX" chunk may or may not be present or may be written in short form, like "s 1" or "seas 1" In any case I would like to have 4 capture groups giving as output : ``` group1 : miniseries group2 : 1 (or None) group3 : 5 group4 : 720p.avi ``` So I've written a regex like this : ``` (^.*)\Ws[eason ]*(\d{1,2}|[ivxlcdm]{1,5})\Wp[art ]*(\d{1,2}|[ivxlcdm]{1,5})\W(.*$) ``` This only works when i have a fully specified filename, including the optional "season XX" string. Is it possible to write a regex that returns "None" as group2 if "season" is not found ?

Accepted Answer

It is easy enough to make the season group optional:

```
(^.*?)(?:\Ws(?:eason )?(\d{1,2}|[ivxlcdm]{1,5}))?\Wp(?:art )?(\d{1,2}|[ivxlcdm]{1,5})\W(.*$)
```

using a non-capturing group (`(?:...)`) plus the 0 or 1 quantifier (`?`). I did have to make the first group non-greedy to prevent it from matching the `season` section of the name.

I also made the `eason` and `art` optional strings into non-capturing optional groups instead of character classes.

Result:

```
>>> import re
>>> p=re.compile(r'(^.*?)(?:\Ws(?:eason )?(\d{1,2}|[ivxlcdm]{1,5}))?\Wp(?:art )?(\d{1,2}|[ivxlcdm]{1,5})\W(.*$)', re.I)
>>> p.search('miniseries.season 1.part 5.720p.avi').groups()
('miniseries', '1', '5', '720p.avi')
>>> p.search('miniseries.part 5.720p.avi').groups()
('miniseries', None, '5', '720p.avi')
>>> p.search('miniseries.part VII.720p.avi').groups()
('miniseries', None, 'VII', '720p.avi')
```

python regex optional capture group

Solution

Problem