How to match patterns "begins with A or ends with B" with Python regular expression?

Question

r'(^|^A)(\S+)(B$|$)'

results to matches everything, which actually equals to ^\S$.

How to write one matches "begins with A or ends with B, may both but not neither?"

PS: I also need refer to group (\S+) in the substring module.

Example:

Match Aanything, anythingB, and refer anything group in the replace.

Answer 1

(^A.*B$)|(^A.*$)|(^.*B$)

Answer 2

Is this the desired behavior?

var rx = /^((?:A)?)(.*?)((?:B)?)$/;
"Aanything".match(rx)
> ["Aanything", "A", "anything", ""]
"anythingB".match(rx)
> ["anythingB", "", "anything", "B"]
"AanythingB".match(rx)
> ["AanythingB", "A", "anything", "B"]
"anything".match(rx)
> ["anything", "", "anything", ""]
"AanythingB".replace(rx, '$1nothing$3');
> "AnothingB"
"AanythingB".replace(rx, '$2');
> "anything"