Question

what’s the quickest way to extract a 5 digit number from a string in c#.

I’ve got

string.Join(null, System.Text.RegularExpressions.Regex.Split(expression, '[^\\d]')); 

Any others?

Answer 1

The regex approach is probably the quickest to implement but not the quickest to run. I compared a simple regex solution to the following manual search code and found that the manual search code is ~2x-2.5x faster for large input strings and up to 4x faster for small strings:

static string Search(string expression) {   int run = 0;   for (int i = 0; i < expression.Length; i++)   {     char c = expression[i];     if (Char.IsDigit(c))       run++;     else if (run == 5)       return expression.Substring(i - run, run);     else       run = 0;   }   return null; } const string pattern = @'\d{5}'; static string NotCached(string expression) {   return Regex.Match(expression, pattern, RegexOptions.Compiled).Value; }  static Regex regex = new Regex(pattern, RegexOptions.Compiled); static string Cached(string expression) {   return regex.Match(expression).Value; } 

Results for a ~50-char string with a 5-digit string in the middle, over 10^6 iterations, latency per call in microseconds (smaller number is faster):

Simple search: 0.648396us

Cached Regex: 2.1414645us

Non-cached Regex: 3.070116us

Results for a ~40K string with a 5-digit string in the middle over 10^4 iterations, latency per call in microseconds (smaller number is faster):

Simple search: 423.801us

Cached Regex: 1155.3948us

Non-cached Regex: 1220.625us

A little surprising: I would have expected Regex — which is compiled to IL — to be comparable to the manual search, at least for very large strings.