Question: I want to pad left every number with zeroes (it has to be 8 digits) in my string.

e.g.

```
asd 123 rete > asd 00000123 rete
4444 my text > 00004444 my text

```

Is it possible to do this using regular expressions? Especially `Regex.Replace()`?

Notice that the number of zeroes is different for different numbers. I mean the padded number has to be 8 digits long.

Answer: 
Thread is old but maybe someone needs this.

Nickon states he wants to use regex. Why? Doesn't matter, maybe its fun. I had to do a inline replace in SQL so some home made SQL functions that calls a C# regex has been helpful.

What I needed to pad looked something like this:

```
abc 1.1.1
abc 1.2.1
abc 1.10.1

```

and I wanted:

```
abc 001.001.001
abc 001.002.001
abc 001.010.001

```

So I could sort it alphabetiacally.

The only solution so far (that I found) was to do the padding and truncating to right length in two steps. I couldn't use a Lambda since this was in SQL and I hadn't prepared my functions for that.

```
//This pads any numbers and truncates it to a length of 8
var unpaddedData = "...";
var paddedData = Regex.Replace(unpaddedData , "(?<=[^\d])(?<digits>\d+)",
                                                     "0000000${digits}");
var zeroPaddedDataOfRightLength = Regex.Replace(paddedData ,"\d+(?=\d{8})","");

```

Explanations:

```
(?<=[^\d])(?<digits>\d+)
(?<=[^\d])       Look behind for any non digit, this is needed if there are 
                 more groups of numbers that needs to be padded
(?<digits>\d+)   Find the numbers and put them in a group named digits to be 
                 used in the replacement pattern

0000000${digits} Pads all the digits matches with 7 zeros

\d+(?=\d{8})     Finds all digits that are followed by at exactly 8 digits. 
                 ?= Doesn't capture the 8 digits.

Regex.Replace(...,"\d+(?=\d{8})","")   
                 Replaces the leading digits with nothing leaving the last 8.

```
```
static void Main(string[] args)
    {
       string myCC = "4556364607935616";
       string myMasked = Maskify(myCC);
       Console.WriteLine(myMasked);
    }        

public static string Maskify(string cc)
    {
       int len = cc.Length;
       if (len <= 4)
          return cc;

       return cc.Substring(len - 4).PadLeft(len, '#');
    }

```

Output:
=======

######5616

Just replace the '#' with 0 or '0'.
Hope that helped :)