[edit]Constants

At most one grammar option can be chosen out ofECMAScript,basic,extended,awk,grep,egrep. If no grammar is chosen,ECMAScript is assumed to be selected. The other options serve as variations, such thatstd::regex("meow", std::regex::icase) is equivalent tostd::regex("meow", std::regex::ECMAScript|std::regex::icase).

[edit]Notes

Because POSIX uses "leftmost longest" matching rule (the longest matching subsequence is matched, and if there are several such subsequences, the first one is matched), it is not suitable, for example, for parsing markup languages: a POSIX regex such as"<tag[^>]*>.*</tag>" would match everything from the first"<tag" to the last"</tag>", including every"</tag>" and"<tag>" in-between. On the other hand, ECMAScript supports non-greedy matches, and the ECMAScript regex"<tag[^>]*>.*?</tag>" would match only until the first closing tag.

[edit]Example

#include <iostream>#include <regex>#include <string> int main(){std::string str="zzxayyzz";std::regex re1(".*(a|xayy)");// ECMAstd::regex re2(".*(a|xayy)", std::regex::extended);// POSIX std::cout<<"Searching for .*(a|xayy) in zzxayyzz:\n";std::smatch m;std::regex_search(str, m, re1);std::cout<<"  ECMA (depth first search) match: "<< m[0]<<'\n';std::regex_search(str, m, re2);std::cout<<"  POSIX (leftmost longest)  match: "<< m[0]<<'\n';}

Searching for .*(a|xayy) in zzxayyzz:  ECMA (depth first search) match: zzxa  POSIX (leftmost longest)  match: zzxayy

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

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