I have a file of several sections, each section start with specific title but all of them ending with the same string, I want to sort the file sections according to the titles without sorting the content of each section (i.e. take the whole section as one block) there is also a blank line between each two section, to clarify the idea if the input is as

string5
z
y
x
string

string2
x
z
y
f
string

the desired output would be as

string2
x
z
y
f
string

string5
z
y
x
string