Intermediate Gotchas

The Search That Accepted a Suffix

a validator that finds a number inside anything

int main() {
  const std::regex whole_number{"[0-9]+"};
  const std::string input = "42ms";

  std::cout << std::boolalpha << std::regex_search(input, whole_number) << '\n';
}

Does the number-only pattern reject "42ms"?

Answer

true

It found the 42; it did not validate the whole input.

Why

std::regex_search succeeds when any subsequence matches the expression. The digits at the front satisfy [0-9]+, leaving the ms suffix irrelevant. std::regex_match instead requires the entire character sequence to match, which is the usual rule for validation. GCC 11.5 emits no warning under -Wall -Wextra because both calls are valid.

The fix

std::regex_match(input, whole_number)

Takeaway: use regex_match for whole-input validation and regex_search only when finding a matching part is the point.

Try it: g++ -std=c++17 main.cpp -o demo && ./demo

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