Regex Constants vs Traits

In C++, std::regex_constants and std::regex_traits are both part of the <regex> library, but they serve different purposes. Here's a breakdown of their differences:

std::regex_constants is a namespace that contains various constants and types used with regular expressions.

These constants control the behavior of regex operations and include flags for syntax options, match behavior, and error types.

Syntax Options: Control the syntax of the regex pattern.

• std::regex_constants::icase: Case-insensitive matching.
• std::regex_constants::ECMAScript: Default syntax (ECMAScript).
• std::regex_constants::basic: Basic POSIX syntax.
• std::regex_constants::extended: Extended POSIX syntax.

Match Flags: Modify the behavior of regex matching functions.

• std::regex_constants::match_default: Default matching behavior.
• std::regex_constants::match_not_bol: Not beginning of line.
• std::regex_constants::match_not_eol: Not end of line.

Error Types: Identify specific regex errors.

• std::regex_constants::error_collate: Collation error.
• std::regex_constants::error_escape: Invalid escape sequence.
• std::regex_constants::error_backref: Invalid back reference.

Example usage:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string text{"Hello WORLD"};
6  std::regex pattern{
7    "hello", std::regex_constants::icase
8  };
9
10  if (std::regex_search(text, pattern)) {
11    std::cout << "Match found";
12  } else {
13    std::cout << "No match";
14  }
15}

1Match found

std::regex_traits is a template class that provides locale-sensitive and character-type-dependent information to the regex engine.

It defines various types and functions used internally by regex objects to handle locale-specific and character-specific operations.

Character Types: Defines character-related types.

• char_type: Character type used by the regex.
• string_type: String type used by the regex.

Locale and Collation: Provides locale-specific information.

• std::locale locale() const: Returns the locale used.
• int value(char_type, int) const: Returns the value of a character in a given base.

Character Classification: Functions to classify characters.

• bool isctype(char_type, char_class_type) const: Checks if a character belongs to a given character class.

Example usage:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::regex_traits<char> traits;
6  std::locale loc =  traits.imbue(
7    std::locale("en_US.UTF-8")
8  );
9  if (traits.isctype('a', std::ctype_base::lower)) {
10    std::cout << "'a' is a lowercase letter";
11  } else {
12    std::cout << "'a' is not a lowercase letter";
13  }
14}

1'a' is a lowercase letter

Purpose:

• std::regex_constants: Provides constants and flags to control regex behavior.
• std::regex_traits: Provides locale-sensitive and character-type-dependent information.

Usage:

• std::regex_constants: Used to modify regex behavior with flags and options.
• std::regex_traits: Used internally by regex objects for locale and character handling.

Understanding these differences helps in effectively using regex in C++ by allowing you to control the behavior and locale-specific aspects of regex operations.