No Matches with C++11 Regex

C++11 regex doesn't match the string

You were specifying POSIX basic regex, in that format you must escape () and {}

I was able to get get matches with a few changes:

 int main(int argc, const char * argv[]){
    using std::cout;
    using std::endl;
    std::regex regexp(R"(\([0-9]\{16\}\):\([0-9]\{5,20\}\):\([a-zA-Z0-9\\+/=]\{28\}\))",std::regex_constants::basic);
    std::smatch match;
    std::string stringified = "1111111111111111:1384537090:Gl21j08WWBDUCmzq9JZoOXDzzP8=";
    if (std::regex_search(stringified, match, regexp)) {
        cout << match[1] << "," << match[2] << "," << match[3]<< endl;
    } else {
        cout << "No matches found" << endl;
    }
    return 0;
}

Or you could use:

std::regex_constants::extended

If you use std::regex_constants::extended you should not escape () and {}

If you don't want to use a raw string, you can do that as well:

std::regex regexp("([0-9]{16}):([0-9]{5,20}):([a-zA-Z0-9\\\\+/=]{28})",std::regex_constants::extended);

You'll just have to double up on the \\ to properly escape them. The above regex also works with the default regex grammar std::regex_constants::ECMAScript

std::regex regexp("([0-9]{16}):([0-9]{5,20}):([a-zA-Z0-9\\\\+/=]{28})");

It looks like GCC just added regex supported in their development branch of GCC 4.9.

C++11 Regex Matching

See gcc's stdc++11 implementation status page -- regexes are not supported as of gcc 4.8

Edit for posterity: As mentioned in the comments, the regex library is now in libstdc++ and should be in gcc 4.9 and on.

C++ regex match, not matching

The regex_match fails when the string doesnt match EXACTLY the pattern. Note that the brd ff:ff:ff:ff:ff:ff part of the string isnt being matched. All you need to do, then, is to append a .* to the pattern:

^\\d{1}:\\s+(\\w+).*?link\\/ether\\s{1}([a-z0-9:]+).*

Also, for that example, the loop isnt necessary. You can use:

if (std::regex_match(line, pieces, interface_address)) {
    std::string name = pieces[1];
    std::string address = pieces[2];
    std::cout << name << address << std::endl;
}

Why doesn't this regex match?

There are two issues with your code, both related to invalid escape-sequences:

1. "\d" is interpreted as an escape-sequence, the resulting string (passed to std::regex) will not contain what you expect — instead use "\\d" to properly get a slash followed by the letter d.

2. "\/" is not a valid escape-sequence, and there really is no need for you to escape /, instead leave it as if ("/").

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Working Example

#include <regex>
#include <iostream>
#include <string>

int main () {
  bool result = std::regex_match (
    std::string ("f 1/1/1 3/3/1 4/4/1"),
    std::regex  ("f \\d+/\\d+/\\d+ \\d+/\\d+/\\d+ \\d+/\\d+/\\d+")
  );

  std::cout << result << std::endl; // 1
}

How to use regex in C++11

If you are compiling with g++ it may be because regex is not fully supported yet. See here for current C++11 status in g++.

C++11 regex matching a full word that does not end with a period?

You need to make sure the word is followed with a word boundary:

std::regex rex(R"(\w+\b(?!\.))");

See the regex demo

Otherwise, backtracking occurs and you find jo in joe. with your pattern.

I also advise to use raw string literals when defining a regex, you get rid of excessive backslashes this way.

Conditionally ignore case in c++11 regular expressions

There are many ways to conditionally set the flag. For example using the conditional operator:

std::regex r(match, ic ? std::regex_constants::icase | std::regex_constants::collate
    : std::regex_constants::collate);

C++11: Safe practice with regex of two possible number of matches

m.size() will always be the number of marked subexpressions in your expression plus 1 (for the whole expression).

In your code you have 4 marked subexpressions, whether these are matched or not has no effect on the size of m.

If you want to now if there are milliseconds, you can check:

m[4].matched

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