C# String replacement using Regex

Question

I currently have the code below, to replace a characters in a string but I now need to replace characters within the first X (in this case 3) characters and leave the rest of the string. In my example below I have 51115 but I need to replace any 5 within the first 3 characters and I should end up with 61115.

My current code:

value = 51115;
oldString = 5;
newString = 6;

result = Regex.Replace(value.ToString(), oldString, newString, RegexOptions.IgnoreCase);

result is now 61116. What would you suggest I do to query just the first x characters?

Thanks

Answer 1

Not particularly fancy, but only give regex the data it should be replacing; only send in the range of characters that should potentially be replaced.

result = Regex.Replace(value.ToString().Substring(0, x), oldString, newString, RegexOptions.IgnoreCase);

Answer 2

If you're just replacing a single character only, you could just write the code to do the replacement yourself. It'd be faster than messing with a substring and then a RegEx replace (which is a waste anyway if you're doing a single-char replacement).

StringBuilder sb = new StringBuilder(oldString.Length);
foreach(char c in oldString) {
  if(c == replaceFrom) { c = replaceTo; }
  sb.Append(c);
}
return sb.ToString();

Answer 3

I love RegEx, but in this case I would just do a .Replace

    string value;
    string oldString;
    string newString;
    value = "51115";
    int iLenToLook;
    iLenToLook = 3;
    oldString = "5";
    newString = "6";
    string result;
result =  value.Length  > iLenToLook ? value.Substring(iLenToLook, value.Length - iLenToLook) :"";

result = value.Substring(0, value.Length >= iLenToLook ? iLenToLook : value.Length).Replace(oldString, newString) + result;

EDIT I changed it to get the non-replaced portion first, in case there were replacement strings of differing lengths than the original.

Answer 4

I'm sure regex whizzes will eventually provide a clean answer, but I think we can build on jcelgin's answer a bit by sprucing it up a bit and wrapping it in a method like this:

string ReplaceInString(string s, string oldString, string newString, int places) {
    return Regex.Replace(s.ToString().Substring(0, places), oldString, newString, RegexOptions.IgnoreCase) 
           + s.ToString().Substring(places);
}

In your example, call like so:

ReplaceInString("51115", "5", "6", 3)

This will return the result you need.

Answer 5

I think the character-by-character option mentioned here is probably clearer, but if you really want a regex:

string result = "";
int value = 55555;
string oldString = "5";
string newString = "6";

var match = new Regex(@"(\d{1,3})(\d+)?").Match(value.ToString());
if (match.Groups.Count > 1)
    result = match.Groups[1].Value.Replace(oldString, newString) + (match.Groups.Count > 2 ? match.Groups[2].Value : "");

Answer 6

Every time someone in the .NET world has a question about regex, I recommend Expresso (link). It's a great tool for working in the confusing and thorny world of regular expressions.