regex - what's the quickest way to extract a 5 digit number from a string in c#

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.