How to use regular expressions to substract all matched strings?

[Fred Lee]

How to use regular expressions to substract all matched strings?
More specifically, I want to generate two variables, one contains all strings begining with AA and ending with BB, the other contains all strings begining with BB and ending with AA

* Example generated by -dataex-. For more info, type help dataex
clear
input str41 var1
"AA: ……BB:……AA: ……BB:……AA:"
end

[Nick Cox]

I think you need to tell us more rules. If I look for the first pattern I find two matches and two other substrings. If I look for the second pattern I find two matches also, but to different substrings, and some junk.

Here is how the first rule parses, for example.

AA: ……BB
:……
AA: ……BB
:……AA:

[Fred Lee]

Exactly, for beginning with AA ending with BB, I want to store the two matches as a variable, namely "First Third"

 
 AA:First BB: Second AA: Third BB: Fourth AA:

[Nick Cox]

That doesn't resolve my uncertainty about what you want to do. Someone else may be able to help.

[Fred Lee]

If you think the colon make you misunderstood, that was my fault. Just drop the colon,

 
 "AA……BB……AA……BB……AA"