Difference between revisions of "cpp/regex"

Revision as of 13:03, 15 March 2018

The regular expressions library provides a class that represents regular expressions, which are a kind of mini-language used to perform pattern matching within strings. 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 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 match_flag_type for the description of supported syntax variations.

Main classes

These classes encapsulate a regular expression and the results of matching a regular expression within a target sequence of characters.

Algorithms

These functions are used to apply the regular expression encapsulated in a regex to a target sequence of characters.

Iterators

The regex iterators are used to traverse the entire set of regular expression matches found within a sequence.

Exceptions

This class defines the type of objects thrown as exceptions to report errors from the regular expressions library.

Traits

The regex traits class is used to encapsulate the localizable aspects of a regex.

Constants

Example

#include <iostream>
#include <iterator>
#include <string>
#include <regex>
 
int main()
{
    std::string s = "Some people, when confronted with a problem, think "
        "\"I know, I'll use regular expressions.\" "
        "Now they have two problems.";
 
    std::regex self_regex("REGULAR EXPRESSIONS",
            std::regex_constants::ECMAScript | std::regex_constants::icase);
    if (std::regex_search(s, self_regex)) {
        std::cout << "Text contains the phrase 'regular expressions'\n";
    }
 
    std::regex word_regex("(\\S+)");
    auto words_begin = 
        std::regex_iterator(s.begin(), s.end(), word_regex);
    auto words_end = std::regex_iterator();
 
    std::cout << "Found "
              << std::distance(words_begin, words_end)
              << " words\n";
 
    const int N = 6;
    std::cout << "Words longer than " << N << " characters:\n";
    for (std::regex_iterator i = words_begin; i != words_end; ++i) {
        std::smatch match = *i;
        std::string match_str = match.str();
        if (match_str.size() > N) {
            std::cout << "  " << match_str << '\n';
        }
    }
 
    std::regex long_word_regex("(\\w{7,})");
    std::string new_s = std::regex_replace(s, long_word_regex, "[$&]");
    std::cout << new_s << '\n';
}

Text contains the phrase 'regular expressions'
Found 19 words
Words longer than 6 characters:
  people,
  confronted
  problem,
  regular
  expressions."
  problems.
Some people, when [confronted] with a [problem], think 
"I know, I'll use [regular] [expressions]." Now they have two [problems].