Keep only numeric value from a string?

Question

I have some strings like this

string phoneNumber = "[phone]";

I would like to strip out the parethenses and the dash, in other word just keep the numeric values.

So the output could look like this

9143951430

How do I get the desired output ?

Answer 1

You do any of the following:

• Use regular expressions. You can use a regular expression with either

  • A negative character class that defines the characters that are what you don't want (those characters other than decimal digits):

    private static readonly Regex rxNonDigits = new Regex( @"[^\d]+");
    

    In which case, you can do take either of these approaches:

    // simply replace the offending substrings with an empty string
    private string CleanStringOfNonDigits_V1( string s )
    {
      if ( string.IsNullOrEmpty(s) ) return s ;
      string cleaned = rxNonDigits.Replace(s, "") ;
      return cleaned ;
    }
    
    // split the string into an array of good substrings
    // using the bad substrings as the delimiter. Then use
    // String.Join() to splice things back together.
    private string CleanStringOfNonDigits_V2( string s )
    {
      if (string.IsNullOrEmpty(s)) return s;
      string cleaned = String.Join( rxNonDigits.Split(s) );
      return cleaned ;
    }
    

  • a positive character set that defines what you do want (decimal digits):

    private static Regex rxDigits = new Regex( @"[\d]+") ;
    

    In which case you can do something like this:

    private string CleanStringOfNonDigits_V3( string s )
    {
      if ( string.IsNullOrEmpty(s) ) return s ;
      StringBuilder sb = new StringBuilder() ;
      for ( Match m = rxDigits.Match(s) ; m.Success ; m = m.NextMatch() )
      {
        sb.Append(m.Value) ;
      }
      string cleaned = sb.ToString() ;
      return cleaned ;
    }
    

• You're not required to use a regular expression, either.

  • You could use LINQ directly, since a string is an IEnumerable<char>:

    private string CleanStringOfNonDigits_V4( string s )
    {
      if ( string.IsNullOrEmpty(s) ) return s;
      string cleaned = new string( s.Where( char.IsDigit ).ToArray() ) ;
      return cleaned;
    }
    

  • If you're only dealing with western alphabets where the only decimal digits you'll see are ASCII, skipping char.IsDigit will likely buy you a little performance:

    private string CleanStringOfNonDigits_V5( string s )
    {
      if (string.IsNullOrEmpty(s)) return s;
      string cleaned = new string(s.Where( c => c-'0' < 10 ).ToArray() ) ;
      return cleaned;
    }
    

• Finally, you can simply iterate over the string, chucking the digits you don't want, like this:

  private string CleanStringOfNonDigits_V6( string s )
  {
    if (string.IsNullOrEmpty(s)) return s;
    StringBuilder sb = new StringBuilder(s.Length) ;
    for (int i = 0; i < s.Length; ++i)
    {
      char c = s[i];
      if ( c < '0' ) continue ;
      if ( c > '9' ) continue ;
      sb.Append(s[i]);
    }
    string cleaned = sb.ToString();
    return cleaned;
  }
  

  Or this:

  private string CleanStringOfNonDigits_V7(string s)
  {
    if (string.IsNullOrEmpty(s)) return s;
    StringBuilder sb = new StringBuilder(s);
    int j = 0 ;
    int i = 0 ;
    while ( i < sb.Length )
    {
      bool isDigit = char.IsDigit( sb[i] ) ;
      if ( isDigit )
      {
        sb[j++] = sb[i++];
      }
      else
      {
        ++i ;
      }
    }
    sb.Length = j;
    string cleaned = sb.ToString();
    return cleaned;
  }
  

From a standpoint of clarity and cleanness of code, the version 1 is what you want. It's hard to beat a one liner.

If performance matters, my suspicion is that the version 7, the last version, is the winner. It creates one temporary — a StringBuilder() and does the transformation in-place within the StringBuilder's in-place buffer.

The other options all do more work.