IF sequences are always consecutive and never interrupted, one could try to grab the first and the last of each sequence and replace each sequence in a loop.

BUT you need to be sure that neither

• neither 43:1:1; 43:1:2; 43:1:5; 43:1:6;
• nor 43:1:1; 43:1:2; 27:3:7; 43:1:3; 43:1:4;

happens.

Still I wouldn't choose to embed Perl into a regex to solve that.

That's only complicating things, because the complex logic would be only obfuscated