Regex nested UL from html in C#

Question

I have a nested ul, li list in my html. How can i get the regex from ul to the end of ul node. In this example below i need to get 2 matches.

First one should be

<ul>
    <li>This is First List</li>
    <li>This is Second List</li>
    <ul>
        <li>This is Second UL First List </li>
        <li>This is Second UL Second List </li>
    </ul>
    <li>This is Third List</li> 
</ul>

and the second one should be

<ul>
        <li>This is Next List</li>
        <ul>
            <li>This is Test </li>
        </ul>
        <li>This is Third List</li> 
        <ul>
            <li>This is Test </li>
        </ul>
 </ul>

My HTML code:

<html>
<p> This is First Paragraph </p>
<ul>
    <li>This is First List</li>
    <li>This is Second List</li>
    <ul>
        <li>This is Second UL First List </li>
        <li>This is Second UL Second List </li>
    </ul>
    <li>This is Third List</li> 
</ul>
<p> This is Second Paragraph </p>   

<ul>
    <li>This is Next List</li>
    <ul>
        <li>This is Test </li>
    </ul>
    <li>This is Third List</li> 
    <ul>
        <li>This is Test </li>
    </ul>
</ul>
</html>

Don't use a regex to parse HTML. See: http://stackoverflow.com/a/1732454/4664094 — James Gould, Apr 05 '17 at 14:33
[Obligatory link](http://stackoverflow.com/a/1732454/2307070) — Thomas Ayoub, Apr 05 '17 at 14:35
You could try HTML Agility Pack (https://htmlagilitypack.codeplex.com). As noted by previous posters don't use RegEx. — P. Kouvarakis, Apr 05 '17 at 14:38
There is also a .NET Core version here: https://github.com/linezero/HtmlAgilityPack — P. Kouvarakis, Apr 05 '17 at 14:46

Scott Weaver · Answer 1 · 2017-04-05T20:04:10.810

You can match nested constructs with .NET Balancing Groups. This feature basically adds the concept of a stack, which can be pushed/popped (<NestedUL>...) and (<-NestedUL>...), and then tested for at the end of the pattern via the last conditional which includes only an empty lookahead guaranteed to fail the pattern (?(NestedUL)(?!)):

var input =
    @"<html>
    <p> This is First Paragraph </p>
    <ul>
        <li>This is First List</li>
        <li>This is Second List</li>
        <ul>
            <li>nested list #1 inside first parent UL</li>
            <li>This is Second UL Second List </li>
        </ul>
        <li>This is Third List</li> 
    </ul>
    <p> This is Second Paragraph </p>   

    <ul>
        <li>This is Next List</li>
        <ul>
            <li>nested list #1 inside second parent UL</li>
        </ul>
        <li>This is Third List</li> 
        <ul>
            <li>nested list #2 inside second parent UL</li>
        </ul>
    </ul>
    </html>";
                var pattern = "<ul>(?:(?<NestedUL><ul>)|(?<-NestedUL></ul>)|.)+?(?(NestedUL)(?!))</ul>";
                var matches = Regex.Matches(input, pattern, RegexOptions.Singleline);
            }

*note the non-greedy quantifier on the repeated alternation +? - if this was greedy, the pattern would happily consume both ul's with a single match.

Regex nested UL from html in C#

1 Answers1