Question

I need to match the following sets of input:

foo_abc_bar  
foo_bar

and get “abc” or an empty string as the result.

So this is the regular expression I wrote:

r'foo_(abc|)[_|]bar'

But for some reason, this does not match with the second string that I have given.

On further inspection, I found that [_|] does not match an empty string.

So, how do I solve this problem?

Answer 1

To make abc_ optional, you could use the question mark operator:

(abc_)?

Thus, the entire regex becomes:

r'foo_(abc_)?bar'

With this regex, the second underscore (if present) will become part of the capture group. If you don’t want that, you could either remove it post-match with .rstrip('_') or use a slightly more complex regex:

r'foo_(?:(abc)_)?bar'

I found that [_|] does not match an empty string.

That’s right. Square brackets denote a character group. The [_|] would match exactly one underscore or exactly one vertical bar, and nothing else. In other words, the vertical bar loses its special meaning when it appears inside a character group.