Regular expression matches the middle content of the specified string

Match two strings A and B. The middle of the string contains A and B: 
Expression: A.*?B ("." means any character, "?" means 0 or more matches) 
Example: Abaidu.comB 
result : Awww.apizl.comB 
matches two strings A and B. The string between A and B contains A but does not contain B: 
Expression: A.*?(?=B) 
Example: Awww.apizl.com B 
Result: Awww.apizl .com 
matches the string between the two strings A and B and does not contain A and B: 
Expression: (?<=A).*?(?=B) 
This kind of writing is not understood, I guess it is if it does not Contains the previous matching character writing (? <= the beginning character to be matched), but does not include the subsequent character writing (? = the ending character to be matched) 
Example: A www.baidu.com B 
Result: www.baidu.com