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

jcelgin · Accepted Answer · 2009-01-29 00:47:56Z

2

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);

answered Jan 29, 2009 at 0:47

jcelgin

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Comments

Ray Hidayat · Accepted Answer · 2009-01-29 00:48:38Z

1

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();

answered Jan 29, 2009 at 0:48

Ray Hidayat

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Comments

flatline · Accepted Answer · 2009-01-29 01:22:15Z

1

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 : "");

edited Jan 29, 2009 at 1:22

answered Jan 29, 2009 at 1:16

flatline

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cmsjr · Accepted Answer · 2009-01-29 01:14:02Z

0

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.

edited Jan 29, 2009 at 1:14

answered Jan 29, 2009 at 1:09

cmsjr

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Comments

jcollum · Accepted Answer · 2009-01-29 01:16:09Z

0

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.

answered Jan 29, 2009 at 1:16

jcollum

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