Question

I’m trying replace two consecutive line breaks with the HTML tag <p/>. So in a string such as:

\r\n\r\n\r\n

There are two consecutive occurrences of \r\n\r\n,

The result should be:

<p/><p/>

but with C# String.Replace, it only detects the first occurrence and I just get back:

<p/>\r\n

So I’m wondering if any regular expression gurus know how to detect that using regular expression?

Edit:

I figured the question is a bit confusing. Let me rephrase it. The requirement should be to replace any "\r\n" with a tag <p/> only if there is another “\r\n” immediately before it.

Such with the string:

\r\n\r\n\r\n

• The first \r\n, does not have another \r\n before it, nothing should be done,
• The second \r\n, it does have another \r\n before it, qualifies for replacement,
• The third \r\n, it does have another \r\n before it, also qualifies for replace.

So the result should be:

<p/><p/>

Answer 1

Yes, you can do this with a regular expression:

string tidyString = Regex.Replace(originalString, @"(?<=\r\n)\r\n", "<p/>");

If performance is an issue then you might find that rebuilding the string manually is quicker than a regex, but the drawback will be the more complicated code. (I’d probably go for the regex in 99% of situations.)

var sb = new StringBuilder(originalString.Length);
int startIndex = 0, nextIndex;
while ((nextIndex = originalString.IndexOf("\r\n", startIndex)) >= 0)
{
    if ((nextIndex == startIndex) && (startIndex > 0))
        sb.Append("<p/>");
    else
        sb.Append(originalString, startIndex, nextIndex - startIndex + 2);

    startIndex = nextIndex + 2;
}
sb.Append(originalString, startIndex, originalString.Length - startIndex);

string tidyString = sb.ToString();