Question

I have string. “12341234115151_log_1.txt” (this string length is not fixed. but “log” pattern always same)
I have a for loop.
each iteration, I want to set the number after “log” of i.

like “12341234115151_log_2.txt”
“12341234115151_log_3.txt”
….

to
“12341234115151_log_123.txt”

in c#, what is a good way to do so?
thanks.

Answer 1

A regex is ideal for this. You can use the Regex.Replace method and use a MatchEvaluator delegate to perform the numerical increment.

string input = "12341234115151_log_1.txt";
string pattern = @"(\d+)(?=\.)";
string result = Regex.Replace(input, pattern,
    m => (int.Parse(m.Groups[1].Value) + 1).ToString());

The pattern breakdown is as follows:

• (\d+): this matches and captures any digit, at least once
• (?=\.): this is a look-ahead which ensures that a period (or dot) follows the number. A dot must be escaped to be a literal dot instead of a regex metacharacter. We know that the value you want to increment is right before the “.txt” so it should always have a dot after it. You could also use (?=\.txt) to make it clearer and be explicit, but you may have to use RegexOptions.IgnoreCase if your filename extension can have different cases.