splitting the merged file using vb.net

You are apparently doing some development along the wrong lines. I remember your previous questions. First you wanted to merge files and had problem with this matter, not you want to split them back, so why merging them? You don't have to explain why. Instead, try to do yourself a great favor: stop this useless activity and get busy with something meaningful. For example, learn some programming, you really need it.

—SA

The simplest way is probably to use a regex:

Dim regex As New Regex("\[(?<filename>[^\]]*)\](?<text>.*)\[/\k<filename>\]", RegexOptions.IgnoreCase Or RegexOptions.Multiline Or RegexOptions.Singleline Or RegexOptions.CultureInvariant Or RegexOptions.IgnorePatternWhitespace Or RegexOptions.Compiled)
Dim ms As MatchCollection = regex.Matches(InputText)</filename></text></filename>

This captures each file into a separate match, each with two groups: "Filename" and "Text".