Multi level .Net Regex

Question

If I have input string in C#, how can I do series of Regex / linq operations on it to do Regex match on one piece of string and then another Regex on all pieces of string which are not matched by 1st Regex.

In other words, for input string:

<!-- Lorem ipsum dolor sit amet, consectetur adipiscing elit -->  
<!-- The quick brown fox jumps over the lazy dog -->
Lorem ipsum dolor sit amet, consectetur adipiscing elit 
The quick brown fox jumps over the lazy  dog
<!-- Lorem ipsum dolor sit amet, consectetur adipiscing elit -->  
<!-- The quick brown fox jumps over the lazy dog -->
Lorem ipsum dolor sit amet, consectetur adipiscing elit
The quick brown fox jumps over the lazy dog

I want to use Regex1 to match lines with <!-- --> and do certain operation on them without parsing them further. And to have Regex2 to match things in pieces of string not matched with Regex1, for example to find all words "fox" and "dog" in those lines and do certain operations on those words.

What is the best way to combine Regex/linq operations in situation like this?

Answer 1

You're in luck since .NET supports variable-length lookbehind.

Therefore, you can use two regexes in sequence.

First, use

^<!--(.*)-->\s*$

to find all comment lines. Backreference $1 will contain whatever is between the delimiters. For example:

Regex paragraphs = new Regex(@"^<!--(.*)-->\s*$", RegexOptions.Multiline);
Match matchResults = paragraphs.Match(subjectString);
while (matchResults.Success) {
    // matched text: matchResults.Value
    // match start: matchResults.Index
    // match length: matchResults.Length
    matchResults = matchResults.NextMatch();

Second, to find and manipulate "dog" and "fox" in the other lines, you can use

(?<!^<!--.*)(dog|fox)

What this regex means is "Match dog or fox unless the line starts with <!--". So if you want to replace them, say, by "cat", use

resultString = Regex.Replace(subjectString, "(?<!^<!--.*)(dog|fox)", "cat", RegexOptions.Multiline);