From cppreference.com
const_referenceoperator[](size_typen)const;(since C++11)If n>0 and n<size(), returns a reference to the
representing the part of the target sequence that was matched by the nth captured
).
If n==0, returns a reference to the
representing the part of the target sequence matched by the entire matched regular expression.
if n>=size(), returns a reference to a
representing an unmatched sub-expression (an empty subrange of the target sequence).
must be true. Otherwise, the behavior is undefined.
Parameters
n - integral number specifying which match to return Return value
Reference to the
representing the specified matched subrange within the target sequence.
Example
Run this code
#include<iostream>#include<regex>#include<string>intmain(){std::stringtarget("baaaby");std::smatchsm;std::regexre1("a(a)*b");std::regex_search(target,sm,re1);std::cout<<"entire match: "<<sm[0]<<'\n'<<"submatch #1: "<<sm[1]<<'\n';std::regexre2("a(a*)b");std::regex_search(target,sm,re2);std::cout<<"entire match: "<<sm[0]<<'\n'<<"submatch #1: "<<sm[1]<<'\n';}Output:
entire match: aaab submatch #1: a entire match: aaab submatch #1: aa See also
returns the sequence of characters for the particular sub-match
(public member function)