C++ std::regex validate integer incorrectly

Question

I have a simple regex express to validate user input is integer. In C# project, i see it validates correctly. Below is my code in C#:

string string_to_validate = Console.ReadLine();    
Regex int_regex = new Regex("[0-9]");
if (int_regex.IsMatch(string_to_validate))
    Console.WriteLine("Regex is match. Validation is success!");
else
    Console.WriteLine("Regex is not match. Validation is fail!");

But in C++ i see it validates in correctly. It only validates correctly string with lengh is 1. Below is my code in C++:

std::string string_to_validate;
std::cin >> string_to_validate;
std::regex int_regex("[0-9]");
if (std::regex_match(string_to_validate,
                     int_regex))
  std::cout << "Regex is match. Validation is success!";
else
  std::cout << "Regex is not match. Validation is fail!";

Please help. It's C++ problem or my problem?

The two regexes are different, what's the value of `string_to_validate`? — Thomas Ayoub, Jul 26 '17 at 09:20
`new Regex("[0-9]")` will find any ASCII digit anywhere in a string. `regex_match` with `"^[0-9]*$"` will only match a string find 0+ ASCII digits in it (an empty string, too). — Wiktor Stribiżew, Jul 26 '17 at 09:25
Sorry about the two regexes are not same, i corrected it. **string_to_validate** is input from user. — Phung Tien Trieu, Jul 26 '17 at 09:32
Replacing std::regex_match with std::regex_search make result is true whether string_to_validate contain numeric character. But i need the result is true if string_to_validate contain only numeric character. — Phung Tien Trieu, Jul 26 '17 at 09:42
Using a regular expression to check that a string consists entirely of digit characters is overkill; this can be done much more simply with a plain old loop and `std::isdigit`. — Pete Becker, Jul 26 '17 at 12:54

ikleschenkov · Accepted Answer · 2017-07-26T10:37:18.570

1

According to MSDN, the C# method bool Regex.IsMatch(String)

Indicates whether the regular expression specified in the Regex constructor finds a match in a specified input string.

So it will return true if there is at least one digit in the input string.

C++ std::regex_match

Determines if the regular expression matches the entire target character sequence

So the whole input string must contains digits to pass the regex.

To validate string with integer of any length in C++ you have to use this regex:

 std::regex int_regex("[0-9]+"); // '+' - quantifier '1 or more' items from range [0-9]

or

 std::regex int_regex("\\d+");

edited Jul 26 '17 at 10:37

answered Jul 26 '17 at 09:34

ikleschenkov

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5
11

std::regex_match return fail with input string "1", "11", "1111". – Phung Tien Trieu Jul 26 '17 at 09:38
@PhungTienTrieu, it's strange. do your test strings have leading/tailing spaces? – ikleschenkov Jul 26 '17 at 09:44
It's ok now. But can you please expain why my original expression work in C# (System.Text.RegularExpression) but does not work in C++ (std::regex, std::regex_match)? – Phung Tien Trieu Jul 26 '17 at 09:57
@PhungTienTrieu I edited the answer with details, why C# regex works, while C++ doesn't – ikleschenkov Jul 26 '17 at 10:38
very helpful. Thank you very much! – Phung Tien Trieu Jul 27 '17 at 09:48

C++ std::regex validate integer incorrectly

1 Answers1