std::regex

Main concept

Almost all operations with regexes can be characterized by operating on several of the following objects:

Target sequence:

• The character sequence that is searched for a pattern.
• This may be a range specified by two iterators, a null-terminated character string or a std::string.

Pattern

• This is the regular expression itself. It determines what constitutes a match.
• It is an object of type std::basic_regex, constructed from a string with special syntax.
• See regex_constants::syntax_option_type for the description of supported syntax variations.

Matched array

• The information about matches may be retrieved as an object of type std::match_results.

Replacement string

• This is a string that determines how to replace the matches
• see regex_constants::match_flag_type for the description of supported syntax variations.

Regex flavor

Six different regular expression flavors or grammars are defined in std::regex_constants:

• ECMAScript: Similar to JavaScript
• basic: Similar to POSIX BRE.
• extended: Similar to POSIX ERE.
• grep: Same as basic, with the addition of treating line feeds as alternation operators.
• egrep: Same as extended, with the addition of treating line feeds as alternation operators.
• awk: Same as extended, with the addition of supporting common escapes for non-printable characters.

Mostly, use the ECMAScript grammar is the default grammar and offers far more features that the other grammars.

Basics - search/match/catch group

std::regex_search(): check whether your regex can match any part of the string.

std::regex_match(): check whether your regex can match the entire subject string.

• std::regex lacks anchors that exclusively match at the start and end of the string, you have to call regex_match() when using a regex to validate user input.

• regex_search() and regex_match() return just true or false.

• To get the parts of the string matched by capturing groups when using either function, you need to pass an object of the template class std::match_results

• std::cmatch when your subject is an array of char

• std::smatch when your subject is an std::string object

• std::wcmatch when your subject is an array of wchar_t

• std::wsmatch when your subject is an std::wstring object

When the function call returns true, you can call the str(), position(), and lengt() member functions of the match_results object to get the text that was matched, or the starting position and its length of the match relative to the subject string.

• Call these member functions without a parameter or with 0 as the parameter to get the overall regex match.
• Call them passing 1 or greater to get the match of a particular capturing group.;
• The size() member function indicates the number of capturing groups plus one for the overall match.

Finding All

To find all regex matches in a string, you need to use an iterator.

• Construct an object of the template class std::regex_iterator using one of these four template instantiations:

  • std::cregex_iterator when your subject is an array of char
  • std::sregex_iterator when your subject is an std::string object
  • std::wcregex_iterator when your subject is an array of wchar_t
  • std::wsregex_iterator when your subject is an std::wstring object

Replacing All

To replace all matches in a string, call std::regex_replace()

• The function returns a new string with the replacements applied.