Replacing only single "\n" in string occurrence

Question

I have a string in C# which can have multiple \n characters. For e.g. :

string tp = "Hello\nWorld \n\n\n !!";

If there is a single occurrence of \n I want to replace it with something, but if more than one \n appear together in the same place I want to leave them alone. So for the tp string above I want to replace the \n between Hello and World, because there is only one at that place, and leave the the three \n nearer the end of the string alone, because they appear in a group.

If I try to use the Replace() method in C# it replaces all of them. How can I resolve this issue?

Re-opened. This was not a duplicate of the chosen answers. It's not about "first" occurence, it's about appearing in a group. Edited the question to make it clearer. — Joel Coehoorn, Dec 21 '18 at 14:35
To actually address the question... I'd normally use RegEx for this, but I'm not sure what that looks like for the \n character in .Net's regex engine, so I'll leave a full answer with code to someone else. — Joel Coehoorn, Dec 21 '18 at 14:37
`string result = Regex.Replace(tp, "\n+", match => match.Value.Length > 1? match.Value : "*");` — Dmitry Bychenko, Dec 21 '18 at 14:38

score 3 · Accepted Answer · answered Dec 21 '18 at 14:38

You can try using regular expressions: let's change \n into "*" whenever \n is single:

using System.Text.RegularExpressions;

... 

string tp = "Hello\nWorld \n\n\n !!";

// "Hello*World \n\n\n !!";
string result = Regex.Replace(tp, "\n+", match => 
  match.Value.Length > 1 
    ? match.Value // multiple (>1) \n in row: leave intact
    : "*");       // single ocurrence: change into "*"

score 0 · Answer 2 · answered Dec 21 '18 at 14:48

0

A solution using loops:

char[] c = "\t"+ tp + "\t".ToCharArray();
for(int i = 1; i < c.Length - 1; i++)
  if(c[i] == '\n' && c[i-1] != '\n' && c[i+1] != '\n')
    c[i] = 'x';
tp = new string(c, 1, c.Length-2);

answered Dec 21 '18 at 14:48

Caius Jard

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score 0 · Answer 3 · answered Dec 21 '18 at 15:04

Use regular expressions and combine negative lookbehind and lookahead:

 var test = "foo\nbar...foo\n\nbar\n\n\nfoo\r\nbar";
 var replaced = System.Text.RegularExpressions.Regex.Replace(test, "(?<!\n)\n(?!\n)", "_");
 // only first and last \n have been replaced

While searching through the input the regex "stops" at any "\n" it finds and verifies if no "\n" is one character behind the current position or ahead. Thus only single "\n" will be replaced.

Replacing only single "\n" in string occurrence

3 Answers3