我可以基于行模式用Regex对文本文件进行分组吗?

给定一个文件:

Timestamp: some text and a number 1
Timestamp: some text and a number 33
Timestamp: some text and a number 1
Timestamp: some text and a number 22
Something totally different, maybe a new pattern
Timestamp: some text and a number 4
Timestamp: some text and a number 2
Something totally different, maybe a new pattern
Something totally different, maybe a new pattern

我想得到第1到4行(TYPE1)和第5行(TYPE2)，第6,7行(TYPE1)和第8,9行(TYPE2)的分组。

这可以在一个正则表达式中完成，或者我应该为每种类型创建一个表达式，然后逐行检查，如果前一行是相同的类型?

最后，我需要返回一个包含pair(int start_char, int end_char)

string[] lines = System.IO.File.ReadAllLines("your taext file");
       var Groups =( 
                from w in lines 
                group w by w[0] into g 
                select new { FirstLetterLine = g.Key, Lins = g });