regex - c# split text file by changing the line number -

i'm trying split text file line numbers, example, if have text file like:

1 ljhgk uygk uygghl \r\n 1 ljhg kjhg kjhg kjh gkj  \r\n 1 kjhl kjhl kjhlkjhkjhlkjhlkjhl  \r\n 2 ljkih lkjhl kjhlkjhlkjhlkjhl  \r\n 2 lkjh lkjh lkjhljkhl  \r\n 3 asdfghjkl  \r\n 3 qweryuiop  \r\n 

i want split 3 parts (1,2,3), how can this? size of text large (~20,000,000 characters) , need efficient way (like regex).

another idea, can use linq groups you're after, splitting each first word. note take each first word, make sure have numbers there. using split/join antipattern, seems work nice here.

var lines = line in s.split("\r\n".tochararray(),                                   stringsplitoptions.removeemptyentries)             let linenumber = line.split(" ".tochararray(), 2).firstordefault()             group line linenumber             g             select string.join("\n", g); 

notes:

• groupby gurenteed return lines in order appeared.
• if block appears more once (e.g. "1 1 2 2 3 3 1"), blocks same number merged.