Combine lines text file (with criteria)

Question

Dear all,

First of all, I am pretty new to C#, but learning quick! :)

At this point I am working on a simple console app.
This app should read through a text file, and when it finds a line that not starts with a certain code (6 options, like ":20:" or ":86:"), it should go one (or more) line back and appends the line without the code to the line with the code.

Example:

input:
:86:1234567890
ABCD
EFGH

output:
:86:1234567890ABCDEFGH

Can someone point me in the correct direction, or maybe post some code?

Thanks in advance!!