Well, the problem has been solved, just don’t use regular expressions...
Sure enough, I was misled by myself because I relied too much on regular expressions.
Attached code:

lst = s1.split('PN')
for index, each in enumerate(lst):
    print index, ' + ', each

Output result:

0  +  
1  +  : 
    abcd
2  +  : 
    abcd
    abcd 
3  +  : 
    abcd
    abcd

Then you only need to write a function and pass in parameters from the outside to get the content between any two PNs...

I don’t understand python, so I wrote js, I don’t know if it will help

var str = 'PN:\nabcd\nPN:\nabcd\nabcd\nPN:\nabcd\nabcd\n';
var result = str.match(/PN:([a-z\n])*[^PN:]+/ig);

Results

["PN:\nabcd\n", "PN:\nabcd\nabcd\n", "PN:\nabcd\nabcd\n"]