C++ Use Regex to find substring

Question

I have a string test

<td><a href="4.%20Functions,%20scope.ppt">4. Functions, scope.ppt</a></td>

I want to find <a href="4.%20Functions,%20scope.ppt"> (as a substring)

As a search with Dr.Google: regex e ("<a href=.*?>"); cmatch =cm; regex e ("<a href=.*?>"); cmatch =cm; to mark substring that I want to find.

How I can do next?

Am I right to use regex_match(htmlString, cm, e); with htmlString as wchar_t*

Answer 1

If you want to find all the matching substrings then you need to use the regex iterators:

// example data
std::wstring const html = LR"(

<td><a href="4.%20Functions,%20scope.ppt">4. Functions, scope.ppt</a></td>
<td><a href="4.%20Functions,%20scope.ppt">4. Functions, scope.ppt</a></td>
<td><a href="4.%20Functions,%20scope.ppt">4. Functions, scope.ppt</a></td>

)";

// for convenience
constexpr auto fast_n_loose = std::regex_constants::optimize|std::regex_constants::icase;

// extract href's
std::wregex const e_link{LR"~(href=(["'])(.*?)\1)~", fast_n_loose};

int main()
{
    // regex iterators       
    std::wsregex_iterator itr_end;
    std::wsregex_iterator itr{std::begin(html), std::end(html), e_link};

    // iterate through the matches
    for(; itr != itr_end; ++itr)
    {
        std::wcout << itr->str(2) << L'\n';
    }
}

Answer 2

This will match the complete a tag and also get the href attribute value,
which is in capture group 2.

It should be done this way because the href attribute can be anywhere in the tag.

<a(?=(?:[^>"']|"[^"]*"|'[^']*')*?\\shref\\s*=\\s*(?:(['"])([\\S\\s]*?)\\1))\\s+(?:"[\\S\\s]*?"|'[\\S\\s]*?'|[^>]*?)+>

You can substitute [\\w:}+ in place of the a tag to get the href from all tags.

https://regex101.com/r/LHZXUM/1

Formatted and tested

 < a                    # a tag, substitute [\w:]+ for any tag

 (?=                    # Asserttion (a pseudo atomic group)
      (?: [^>"'] | " [^"]* " | ' [^']* ' )*?
      \s href \s* = \s* 
      (?:
           ( ['"] )               # (1), Quote
           ( [\S\s]*? )           # (2), href value
           \1 
      )
 )
 \s+ 
 (?: " [\S\s]*? " | ' [\S\s]*? ' | [^>]*? )+
 >