How to check which matching group was used to match (boost-regex)

Question

I'm using boost::regex to parse some formatting string where '%' symbol is escape character. Because I do not have much experience with boost::regex, and with regex at all to be honest I do some trial and error. This code is some kind of prototype that I came up with.

std::string regex_string = 
            "(?:%d\\{(.*)\\})|"                   //this group will catch string for formatting time
            "(?:%([hHmMsSqQtTlLcCxXmMnNpP]))|"    //symbols that have some meaning
            "(?:\\{(.*?)\\})|"                    //some other groups
            "(?:%(.*?)\\s)|"
            "(?:([^%]*))";

    boost::regex regex;
    boost::smatch match;

    try
    {
        regex.assign(regex_string, boost::regex_constants::icase);
        boost::sregex_iterator res(pattern.begin(), pattern.end(), regex);
        //pattern in line above is string which I'm parsing
        boost::sregex_iterator end;
        for(; res != end; ++res)
        {
            match = *res;
            output << match.get_last_closed_paren();
            //I want to know if the thing that was just written to output is from group describing time string
            output << "\n";
        }


    }
    catch(boost::regex_error &e)
    {
        output<<"regex error\n";
    }

And this works pretty good, on the output I have exactly what I want to catch. But I do not know from which group it is. I could do something like match[index_of_time_group]!="" but this is kind of fragile, and doesn't look too good. If I change regex_string index that was pointing on group catching string for formatting time could also change.

Is there a neat way to do this? Something like naming groups? I'll be grateful for any help.

score 1 · Accepted Answer · edited Nov 28 '12 at 20:18

1

You can use boost::sub_match::matched bool member:

if(match[index_of_time_group].matched) process_it(match);

It is also possible to use named groups in regexp like: (?<name_of_group>.*), and with this above line could be changed to:

if(match["name_of_group"].matched) process_it(match);

edited Nov 28 '12 at 20:18

Dino

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answered Nov 28 '12 at 19:48

Rost

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Thanks! After a little tweak I came up with following solution. I changed part of regexp: `(?:%d\\{(?.*)\\})`, so now it is named group. With this I could use code you proposed like: `if(match["time"].matched) output<<"It was time string after all";`. Probably I'll do some more tweaking along the way, but I'll take liberty to edit your post and mark it as my answer. ;) Thanks again. – Dino Nov 28 '12 at 20:10

score 0 · Answer 2 · answered Nov 28 '12 at 19:52

Dynamically build regex_string from pairs of name/pattern, and return a name->index mapping as well as the regex. Then write some code that determines if the match comes from a given name.

If you are insane, you can do it at compile time (the mapping from tag to index that is). It isn't worth it.

How to check which matching group was used to match (boost-regex)

2 Answers2