This regex doesn't work in c++

Question

It is supposed to match "abababab" since "ab" is repeated more than two times consecutively but the code isn't printing any output. Is there some other trick in using regex in C++.

I tried with other languages and it works just fine.

#include<bits/stdc++.h>

int main(){

  std::string s ("xaxababababaxax");
  std::smatch m;
  std::regex e ("(.+)\1\1+");   

   while (std::regex_search (s,m,e)) {
    for (auto x:m) std::cout << x << " ";
    std::cout << std::endl;
    s = m.suffix().str();
  }

  return 0;
}

Isn't (.+) just greedy and matches the whole string? Therefore the \1 does not make any sense — Ed Heal
– Ed Heal, Commented Jun 28, 2015 at 10:46
Don't use <bits/...>, it's not portable and you have no guarantee it will work tomorrow. Also, you may need to upgrade gcc to the latest and greatest 5.x version, earlier ones don't have a working regexp implementation. — n. m. could be an AI
– n. m. could be an AI, Commented Jun 28, 2015 at 10:48
Use a raw string R"((.+)\1\1+)" or escape the backslashes "(.+)\\1\\1+" — Lucas Trzesniewski
– Lucas Trzesniewski, Commented Jun 28, 2015 at 10:50
@Ed, backtracking will make .+ match successively less until the \1 matches. It's a fairly common idiom in regex. — Toby Speight
– Toby Speight, Commented Apr 11, 2018 at 8:06

Spacemoose · Accepted Answer · 2015-06-28 10:57:06Z

6

Your problem is your backslashes are escaping the '1''s in your string. You need to inform std::regex to treat them as '\' 's. You can do this by using a raw string R"((.+)\1\1+)", or by escaping the slashes, as shown here:

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


int main(){

  std::string s ("xaxababababaxax");
  std::smatch m;
  std::regex e ("(.+)\\1\\1+");

   while (std::regex_search (s,m,e)) {
    for (auto x:m) std::cout << x << " ";
    std::cout << std::endl;
    s = m.suffix().str();
  }

  return 0;
}

Which produces the output

abababab ab

answered Jun 28, 2015 at 10:57

Spacemoose

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3 Comments

user2798694 Over a year ago

How do I add a flag to return the minimum string that matches for example aaaaaa should match a repeated 6 times not aa repeated three times?

Spacemoose Over a year ago

Hmm. I have to admit you're doing stuff with regex's I haven't tried before. You could try using the "non-greedy-operator" stackoverflow.com/questions/11898998/regex-match-non-greedy , which is '?', and see if that gets the results you want. Let me know what you find out!

Spacemoose Over a year ago

No problem. Regex's can be a pain in the butt, but they are a really worthwhile skill to develop.

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