How do I replace a specific occurrence of a string in a string?

Question

I have a string which may contain "title1" twice in it.

e.g.

server/api/shows?title1=its always sunny in philadelphia&title1=breaking bad ...

I need to change the second instance of the word "title1" to "title2"

I already know how to identify whether there ARE two instances of the string in the string.

int occCount = Regex.Matches(callingURL, "title1=").Count;

if (occCount > 1)
{
     //here's where I need to replace the second "title1" to "title2"
}

I know we can probably use Regex here but I'm not able to get the replace on the second instance. Can anyone give me a hand?

if there are 3 instances of the word `title1` do you need to change the third one to `title3`? — Sam I am says Reinstate Monica, Jun 26 '13 at 16:40
I have checks to ensure the string will never have more than 2 instances. the outer string gets built on user selections on the screen and i account for the correct selections... — JJ., Jun 26 '13 at 16:43

score 15 · Accepted Answer · answered Jun 26 '13 at 16:45

15

This will only replace the second instance of title1 (and any subsequent instances) after the first:

string output = Regex.Replace(input, @"(?<=title1.*)title1", "title2");

However, if there are more than 2 instances, it may not be what you want. It's a little crude, but you can do this to handle any number of occurrences:

int i = 1;
string output = Regex.Replace(input, @"title1", m => "title" + i++);

answered Jun 26 '13 at 16:45

p.s.w.g

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pswg, I know I already accepted your answer but I just tried to do exactly the same thing but with the word "episodetitle1" and it's not giving me the same results. Care to give another hand? callingURL = Regex.Replace(callingURL, @"(?<=episodetitle1.*)episodetitle1", "episodetitle2"); – JJ. Jun 26 '13 at 17:05
@Rj. What's the input string? – p.s.w.g Jun 26 '13 at 17:10
something like this: server/api/Shows?title1=breaking%20bad&title2=two%20and%20a%20half%20men&episodetitle1=pilot&episodetitle2=a%20big%20bag%20of%20dog – JJ. Jun 26 '13 at 17:35
@Rj. Well it looks like that string has already had the second `episodetitle1` replaced with `episodetitle2`, but in any case, the pattern works for me whether you use `title1` or `episodetitle1`. – p.s.w.g Jun 26 '13 at 17:43

score 3 · Answer 2 · answered Apr 12 '17 at 08:08

You can use the regex replace MatchEvaluator and give it a "state":

string callingURL = @"server/api/shows?title1=its always sunny in philadelphia&title1=breaking bad";

int found = -1;
string callingUrl2 = Regex.Replace(callingURL, "title1=", x =>
{
    found++;
    return found == 1 ? "title2=" : x.Value;
});

The replace can be one-lined by using the postfixed ++ operator (quite unreadable).

string callingUrl2 = Regex.Replace(callingURL, "title1=", x => found++ == 1 ? "title2=" : x.Value);

score 2 · Answer 3 · answered Jun 26 '13 at 16:47

2

you can specify a count, and an index to start searching at

string str = @"server/api/shows?title1=its always sunny in philadelphia&title1=breaking bad ...";

Regex regex = new Regex(@"title1");
str = regex.Replace(str, "title2", 1, str.IndexOf("title1") + 6);

answered Jun 26 '13 at 16:47

Sam I am says Reinstate Monica

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This is a good approach, but why would you use a regular expression in the last step? You could simply do a `string.Replace`. – p.s.w.g Jun 26 '13 at 17:22
@p.s.w.g does `string.Replace` have a field specifying where you start the search? – Sam I am says Reinstate Monica Jun 26 '13 at 17:25
Nope, you're right. You'd have to do a couple of `string.Substring`'s first (like a one answer had, which is now deleted). – p.s.w.g Jun 26 '13 at 17:29
Just what I needed - couldn't find this in static API. I'm disappointed that the static method of same name is missing this basic feature; it tricked me into believing that the API lacked it. IMHO great example of how you shouldn't overload method names if you're going to re-implement the static / non-static method with different feature sets. – Adam Jul 18 '15 at 22:34

score 1 · Answer 4 · answered Jun 26 '13 at 16:44

1

You could perhaps make use of a negative lookahead:

title1(?!.*title1)

And replace with title2.

See how it's working here.

answered Jun 26 '13 at 16:44

Jerry

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@Anirudh The user already made sure with the `if` that there are two instances of `title1`. – Jerry Jun 26 '13 at 16:58
it can have a single occurance – Anirudha Jun 26 '13 at 17:24

score 0 · Answer 5 · edited May 23 '17 at 12:32

0

I found this link immediately on a google search.

C# - indexOf the nth occurrence of a string?

Get the IndexOf the first occurrence of the string.

Use the returned IndexOf's startIndex +1 for the starting position of the second IndexOf.

Substring it into two strings at the appropriate index of the "1" character.

Concat it back together with the "2" character.

edited May 23 '17 at 12:32

Community

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answered Jun 26 '13 at 16:56

deegee

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score 0 · Answer 6 · answered Jun 26 '13 at 18:16

The way P.S.W.G did is really awesome. but below I mentioned one simple way to get it done for those who do have problem in lambda and regex expression..;)

int index = input.LastIndexOf("title1=");

string output4 = input.Substring(0, index - 1) + "&title2" + input.Substring(index + "title1".Length, input.Length - index - "title1".Length);

How do I replace a specific occurrence of a string in a string?

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