Formatting Dates with Regex

Using regex to format dates in C++ involves capturing the different parts of a date string and rearranging them. For example, converting a date from MM/DD/YYYY to YYYY/MM/DD.

Example: MM/DD/YYYY to YYYY/MM/DD

Here’s how to achieve this with std::regex_replace():

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string input = "12/31/2023";
6  std::regex pattern(
7    R"((\d{2})/(\d{2})/(\d{4}))"
8  );
9  std::string replacement = "$3/$1/$2";
10
11  std::string result = std::regex_replace(
12    input, pattern, replacement);
13  std::cout << "Original: " << input
14            << "\nFormatted: " << result;
15}

1Original: 12/31/2023
2Formatted: 2023/12/31

In this example, (\d{2}) captures the month and day, while (\d{4}) captures the year. The replacement string $3/$1/$2 rearranges the captured groups.

More Complex Formatting

For more complex date formats, you can extend the regex pattern and replacement string. For example, converting MM-DD-YYYY to DD.MM.YYYY:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string input = "12-31-2023";
6  std::regex pattern(
7    R"((\d{2})-(\d{2})-(\d{4}))");
8  std::string replacement = "$2.$1.$3";
9
10  std::string result = std::regex_replace(
11    input, pattern, replacement);
12  std::cout << "Original: " << input
13            << "\nFormatted: " << result;
14}

1Original: 12-31-2023
2Formatted: 31.12.2023

Using Named Capture Groups

C++ does not support named capture groups directly, but you can achieve similar results with careful grouping and replacements.

Summary

Regex in C++ provides a flexible way to reformat date strings. By capturing the different components of the date and rearranging them in the replacement string, you can easily convert between different date formats.