(?=) is used to check if a string (which we want to match) is followed by another string or not.
(?<=) is used to check if a string (which we want to match) is preceded by another string or not.

Let’s see an example for (?=) first.
We have a line Hello World . And we want to match Hello if it is followed by World .

Regex will be

Hello(?=\sWorld)

Output :
Hello World

Pattern Explanation :
Hello :  Actual string to match.
(?= :  Represents the starting of positive lookahead.
\s :  Matches any whitespace character like space, tab, newline etc.
World :  Matches the string which should be followed by Hello.
) :  Represents ending of the positive lookahead.

If we use (?<=) in place of (?=) here, it will not match Hello .
If we wanted to match World which is preceded by Hello then we could use (?<=)

Then regex will be

(?<=Hello\s)World

Output :
Hello World

Pattern Explanation :
(?<= :  Starting of Positive Lookbehind.
Hello :  String that should be preceded by.
\s :  Matches any whitespace character like space, tab, newline etc.
) :  Ending of Positive Lookbehind.
World :  Actual string to match.

We can not use (?=) instead of (?<=) here for matching World which is preceded by Hello .