Overlapping matches in Regex

c# regex overlap
jevakallio picture jevakallio · Nov 26, 2008 · Viewed 11.6k times · Source

I can't seem to find an answer to this problem, and I'm wondering if one exists. Simplified example:

Consider a string "nnnn", where I want to find all matches of "nn" - but also those that overlap with each other. So the regex would provide the following 3 matches:

  1. nnnn
  2. nnnn
  3. nnnn

I realize this is not exactly what regexes are meant for, but walking the string and parsing this manually seems like an awful lot of code, considering that in reality the matches would have to be done using a pattern, not a literal string.

Answer

Jan Goyvaerts picture Jan Goyvaerts · Nov 26, 2008

Using a lookahead with a capturing group works, at the expense of making your regex slower and more complicated. An alternative solution is to tell the Regex.Match() method where the next match attempt should begin. Try this:

Regex regexObj = new Regex("nn");
Match matchObj = regexObj.Match(subjectString);
while (matchObj.Success) {
    matchObj = regexObj.Match(subjectString, matchObj.Index + 1); 
}

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