C# how to separate a string by numbering (1. 2. ...)

You can use

string[] result = Regex.Split(text, @"(?!^)(?=(?<!\d)\d+:)")

See this regex demo. Note that the (?<!\d) negative lookbehind is necessary when you have bullet point with two or more digits. Details:

• (?!^) - not at the start of string
• (?=(?<!\d)\d+:) - the position that is immediately followed with one or more digits (not preceded with any digit) and a : char.

If you use a capture group () like this:

string[] result = Regex.Split(str, @"(\d+:)");

the captured values will be added to the array too. Then all that is left to do is to merge every first value with every second value (we skip index 0 as it is empty):

List<string> values = new();
for (int i = 1; i < result.Length; i += 2)
{
    values.Add(result[i] + result[i + 1]); 
}

There are probably cleaner ways to do this, but this works.

Using \D+ matches 1 or more non digits, and will therefore match : This to split on.

Instead of using split, you can also match the parts:

\b[0-9]+:.*?(?=\b[0-9]+:|$)

The pattern matches:

• \b A word boundary to prevent a partial word match
• [0-9]+: Match 1+ digits and :
• .*? Match as least as possible characters
• (?=\b[0-9]+:|$) Positive lookahead, assert either 1+ digits and : or the end of the string to the right

.NET regex demo

Example in C#:

string str = "1: This 2: Is 3: A 4: Test";
string pattern = @"\b[0-9]+:.*?(?=\b[0-9]+:|$)";

MatchCollection matchList = Regex.Matches(str, pattern);
string[] result = matchList.Cast<Match>().Select(match => match.Value).ToArray();

Array.ForEach(result, Console.WriteLine);

Output

1: This 
2: Is 
3: A 
4: Test