Find and extract numbers from a string c#

<pre class="lang-cs prettyprint prettyprinted"> private string GetNumberFromString(string sLongString, int iLimitNumbers)
 {
 string sReturn = "NA";
 int iNumbersCounter = 0;
 int iCharCounter = 0; 

 string sAlphaChars = string.Empty;
 string sNumbers = string.Empty;
 foreach (char str in sLongString)
 {
 if (char.IsDigit(str))
 {
 sNumbers += str.ToString();
 iNumbersCounter++;
 if (iNumbersCounter == iLimitNumbers)
 {
 return sReturn = sNumbers;
 }
 }
 else
 {
 sAlphaChars += str.ToString();
 iCharCounter++;
 // reset the counter 
 iNumbersCounter = 0; 
 }
 }
 return sReturn;
 }</pre>