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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?

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4  
if there are 3 instances of the word title1 do you need to change the third one to title3? – Sam I am 7 hours ago
This sounds like a broader problem to me. – Dan Teesdale 7 hours ago
it will NEVER have more than 2 instances – Rj. 7 hours ago
1  
you know the old saying NEVER SAY NEVER – DJ KRAZE 7 hours ago
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... – Rj. 7 hours ago

5 Answers

up vote 7 down vote accepted

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++);
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im testing this now – Rj. 7 hours ago
this did the trick. thank you sir! – Rj. 7 hours ago
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"); – Rj. 6 hours ago
@Rj. What's the input string? – p.s.w.g 6 hours ago
something like this: server/api/Shows?title1=breaking%20bad&title2=two%20and%20a%20half%20men&episode‌​title1=pilot&episodetitle2=a%20big%20bag%20of%20dog – Rj. 6 hours ago
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up vote 1 down vote

You could perhaps make use of a negative lookahead:

title1(?!.*title1)

And replace with title2.

See how it's working here.

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what if it has a single title..you would replace wrong one – Anirudh 7 hours ago
@Anirudh The user already made sure with the if that there are two instances of title1. – Jerry 7 hours ago
he says it will never have more than 2 instances.. – Anirudh 7 hours ago
Um.... So what? – Jerry 7 hours ago
it can have a single occurance – Anirudh 6 hours ago
up vote 1 down vote

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);
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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 6 hours ago
@p.s.w.g does string.Replace have a field specifying where you start the search? – Sam I am 6 hours ago
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 6 hours ago
up vote 0 down vote

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.

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up vote 0 down vote

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);

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