Replace Contiguous Instance of a String in C#

Question

How can I replace contiguous substring of a string in C#? For example, the string

"<p>The   quick   fox</p>"

will be converted to

"<p>The quick fox</p>"

It wasn't necessarily of variable length. I just want to replace all contiguous to just one; — rajeemcariazo, Feb 05 '15 at 12:35
@Thomas I tried google but i can't find the right keywords, and all the results seem to be for contiguous single characters only using regex — rajeemcariazo, Feb 05 '15 at 12:37
possible duplicate of [How do I replace multiple spaces with a single space in C#?](http://stackoverflow.com/questions/206717/how-do-i-replace-multiple-spaces-with-a-single-space-in-c) — Gusdor, Feb 05 '15 at 13:34
@Gusdor this is not a duplicate because what I am trying to replace here are strings, not single characters, they have different approach in Regex — rajeemcariazo, Feb 05 '15 at 14:10

score 3 · Answer 1 · answered Feb 05 '15 at 12:34

3

Use the below regex

@"(.+)\1+"

(.+) captures the group of characters and matches also the following \1+ one or more same set of characters.

And then replace the match with $1

DEMO

string result = Regex.Replace(str, @"(.+)\1+", "$1");

answered Feb 05 '15 at 12:34

Avinash Raj

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I don't get why (.+) matches exactly &nbsp and nothing else in this example, could you explain that? (I verified it with a regex tester - i just don't understand why it behaves like it does) – H W Feb 05 '15 at 12:42
1

it actually replaces all contiguous strings – rajeemcariazo Feb 05 '15 at 12:43
how if i only want to replace contiguous – rajeemcariazo Feb 05 '15 at 12:43
then use this `( )\1+` regex and replace the match by `$1` , https://regex101.com/r/tJ4cY2/2 – Avinash Raj Feb 05 '15 at 12:45
Or forget the backreference entirely... `( )+`. `+` is already one-or-more and greedy. – Gusdor Feb 05 '15 at 13:28
note that your regex also replaces ` ` with ` ` . It decreases the performance little bit. – Avinash Raj Feb 05 '15 at 13:39

Jonny 5 · Answer 2 · 2015-02-05T13:02:33.107

2

Maybe this simple one is enough:

(&nbsp;){2,}

and replace with $1 (  that's captured in first parenthesized group)

See test at regex101

To check, if a substring is followed by itself, also can use a lookahead:

(?:(&nbsp;)(?=\1))+

and replace with empty. See test at regex101.com

edited Feb 05 '15 at 13:02

answered Feb 05 '15 at 12:39

Jonny 5

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schnaader · Answer 3 · 2015-02-06T07:31:06.630

2

Let's call the original string s and the substring subString:

    var s = "<p>The&nbsp;&nbsp;&nbsp;quick&nbsp;&nbsp;&nbsp;fox</p>";
    var subString = "&nbsp;";

I'd prefer this instead of a regex, much more readable:

    var subStringTwice = subString + subString;

    while (s.Contains(subStringTwice))
    {
        s = s.Replace(subStringTwice, subString);
    }

Another possible solution with better performance:

    var elements = s.Split(new []{subString}, StringSplitOptions.RemoveEmptyEntries);
    s = string.Join(subString, elements);
    // This part is only needed when subString can appear at the start or the end of s
    if (result != "")
    {
        if (s.StartsWith(subString)) result = subString + result;
        if (s.EndsWith(subString)) result = result + subString;                
    }

edited Feb 06 '15 at 07:31

answered Feb 05 '15 at 12:42

schnaader

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To my regex loving eyes, this is only more readable if you are **not familiar with regex**. Algorithmically speaking, this could also be much more expensive but that is barely worth mentioning. – Gusdor Feb 05 '15 at 13:26
Yup... repeated string replace is expensive, and the split method fails if the substring appears at the start or end of the input. – Rawling Feb 05 '15 at 15:40
@Rawling: Thanks for catching that, fixed. – schnaader Feb 06 '15 at 07:31

Replace Contiguous Instance of a String in C#

3 Answers3