How to to remove characters from numeric value using regex

How about without regex?

var s = "4,59,999/-";
var array = s.Where(c => char.IsDigit(c)).ToArray(); 

or shorter

var array = s.Where(char.IsDigit).ToArray(); 

And you can use this array in a string(Char[]) constructor.

var result = new string(array); // 459999

You don't need regex, you could use:

 textBox2.Text = String.Concat(s.Where(Char.IsDigit));

Much better is to use decimal.Parse/TryParse:

string s = "4,59,999/-.";
decimal price;
if (decimal.TryParse(s.Split('/')[0], NumberStyles.Currency, NumberFormatInfo.InvariantInfo, out price))
    textBox2.Text = price.ToString("G");

Just replace with empty string.

string value = Regex.Replace(s, @"[^\d]", ""); // See the change in the replace string.
textBox2.Text= value;

Note You don't require the if as the regex replace will work only if there is a match for non digits ([^\d])

Should just be a case of replacing it with an empty string instead of a space?

Regex.Replace(s, @"[^\d]", String.Empty);

Currently you're replacing these characters with a space. Use an empty set of quotes instead.

if (Regex.IsMatch(s,@"\b[1-9]\d*(?:,[0-9]+)*/?-?"))
{
    string value = Regex.Replace(s, @"[^\d]", "");
    textBox2.Text= value;
} 

You are replacing the ",", "/", "-" and " " with a white space. Try this instead:

string value = Regex.Replace(s, @"[^\d]", "");

Hope this helps.

Linq is a possible solution:

  String s = "4,59,999/-";
  ...
  textBox2.Text = new String(s.Where(item => item >= '0' && item <= '9').ToArray());

Try something like this it will work 100% in php so just change some syntax for C#

<?php
    $str = "4,59,999/-.";
    echo $str = preg_replace('/[^a-z0-9]/', '', $str);
?>