How do I combine multiple regex patterns into one in C++?

Question

Ryan McCombe · Accepted Answer

Combining multiple regex patterns into one in C++ can be useful when you need to match any one of several patterns. Here are some common ways to achieve this:

Using the Alternation Operator (`|`)

The most straightforward way to combine multiple regex patterns is to use the alternation operator |, which matches any one of the separated patterns.

For example, suppose you want to match either "cat" or "dog":

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string text{"I have a cat and a dog"};
6  std::regex pattern{"cat|dog"};
7
8  auto words_begin = std::sregex_iterator(
9    text.begin(), text.end(), pattern);
10  auto words_end = std::sregex_iterator();
11
12  for (
13    std::sregex_iterator i = words_begin;
14    i != words_end;
15    ++i
16  ) {
17    std::smatch match = *i;
18    std::cout << "Match: "
19      << match.str() << "\n";
20  }
21}

1Match: cat
2Match: dog

Using Non-Capturing Groups

If you want to combine more complex patterns and avoid capturing groups in your matches, you can use non-capturing groups (?: ...) to group patterns without affecting the match results.

Below, we match either "cat", "dog", or "fish" without capturing the groups:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string text{
6    "I have a cat, a dog, and a fish"};
7  std::regex pattern{"(?:cat|dog|fish)"};
8
9  auto words_begin = std::sregex_iterator(
10    text.begin(), text.end(), pattern
11  );
12  auto words_end = std::sregex_iterator();
13
14  for (
15    std::sregex_iterator i = words_begin;
16    i != words_end;
17    ++i
18  ) {
19    std::smatch match = *i;
20    std::cout << "Match: "
21      << match.str() << "\n";
22  }
23}

1Match: cat
2Match: dog
3Match: fish

Combining Patterns with Common Prefixes

If the patterns share common prefixes, you can optimize by combining them more efficiently. For instance, to match "apple", "apricot", or "apartment", you can use a combined pattern:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string text{
6    "I like apples, apricots, and apartments"
7  };
8  std::regex pattern{"ap(?:ple|ricot|artment)"};
9
10  auto words_begin = std::sregex_iterator(
11    text.begin(), text.end(), pattern);
12  auto words_end = std::sregex_iterator();
13
14  for (
15    std::sregex_iterator i = words_begin;
16    i != words_end;
17    ++i
18  ) {
19    std::smatch match = *i;
20    std::cout << "Match: "
21      << match.str() << "\n";
22  }
23}

1Match: apples
2Match: apricots
3Match: apartments

Using Raw String Literals for Complex Patterns

For complex patterns that involve a lot of special characters, using raw string literals can make the regex more readable:

1#include <iostream>
2#include <regex>
3
4int main() {
5  std::string text{
6    "I have a cat, a dog, and a fish"
7  };
8
9  std::regex pattern{
10    R"((cat|dog|fish))" // Raw string literal
11  };  
12
13  auto words_begin = std::sregex_iterator(
14    text.begin(), text.end(), pattern);
15  auto words_end = std::sregex_iterator();
16
17  for (
18    std::sregex_iterator i = words_begin;
19    i != words_end;
20    ++i
21  ) {
22    std::smatch match = *i;
23    std::cout << "Match: "
24      << match.str() << "\n";
25  }
26}

1Match: cat
2Match: dog
3Match: fish

By using these techniques, you can combine multiple regex patterns into one, making your pattern matching more efficient and easier to manage.

Regular Expressions

Combine Multiple Regex

Using the Alternation Operator (`|`)

Using Non-Capturing Groups

Combining Patterns with Common Prefixes

Using Raw String Literals for Complex Patterns

Regular Expressions

Professional C++

Questions & Answers

Combine Multiple Regex

Using the Alternation Operator (|)

Using Non-Capturing Groups

Combining Patterns with Common Prefixes

Using Raw String Literals for Complex Patterns

Regular Expressions

Questions & Answers

Using the Alternation Operator (`|`)