I am new to c++ regex. I have a string "{1,2,3}" and I want to extract the numbers 1 2 3. I thought I should use regex_search but it failed.
#include<iostream>
#include<regex>
#include<string>
using namespace std;
int main()
{
string s1("{1,2,3}");
string s2("{}");
smatch sm;
regex e(R"(\d+)");
cout << s1 << endl;
if (regex_search(s1,sm,e)){
cout << "size: " << sm.size() << endl;
for (int i = 0 ; i < sm.size(); ++i){
cout << "the " << i+1 << "th match" <<": "<< sm[i] << endl;
}
}
}
The result:
{1,2,3}
size: 1
the 1th match: 1
std::regex_search returns after only the first match found.
What std::smatch gives you is all the matched groups in the regular expression. Your regular expression only contains one group so std::smatch only has one item in it.
If you want to find all matches you need to use std::sregex_iterator.
int main()
{
std::string s1("{1,2,3}");
std::regex e(R"(\d+)");
std::cout << s1 << std::endl;
std::sregex_iterator iter(s1.begin(), s1.end(), e);
std::sregex_iterator end;
while(iter != end)
{
std::cout << "size: " << iter->size() << std::endl;
for(unsigned i = 0; i < iter->size(); ++i)
{
std::cout << "the " << i + 1 << "th match" << ": " << (*iter)[i] << std::endl;
}
++iter;
}
}
Output:
{1,2,3}
size: 1
the 1th match: 1
size: 1
the 1th match: 2
size: 1
the 1th match: 3
The end
iterator is default constructed by design so that it is equal to iter
when iter
has run out of matches. Notice at the bottom of the loop I do ++iter
. That moves iter
on to the next match. When there are no more matches, iter
has the same value as the default constructed end
.
Another example to show the submatching (capture groups):
int main()
{
std::string s1("{1,2,3}{4,5,6}{7,8,9}");
std::regex e(R"~((\d+),(\d+),(\d+))~");
std::cout << s1 << std::endl;
std::sregex_iterator iter(s1.begin(), s1.end(), e);
std::sregex_iterator end;
while(iter != end)
{
std::cout << "size: " << iter->size() << std::endl;
std::cout << "expression match #" << 0 << ": " << (*iter)[0] << std::endl;
for(unsigned i = 1; i < iter->size(); ++i)
{
std::cout << "capture submatch #" << i << ": " << (*iter)[i] << std::endl;
}
++iter;
}
}
Output:
{1,2,3}{4,5,6}{7,8,9}
size: 4
expression match #0: 1,2,3
capture submatch #1: 1
capture submatch #2: 2
capture submatch #3: 3
size: 4
expression match #0: 4,5,6
capture submatch #1: 4
capture submatch #2: 5
capture submatch #3: 6
size: 4
expression match #0: 7,8,9
capture submatch #1: 7
capture submatch #2: 8
capture submatch #3: 9
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With