Truncate on first non-printable character-CodePudding

I am trying to use std::regex to truncate on the first non-printable character.

I tried

  std::string ExtractPrintableString(const std::string& message) {
    std::regex trim_nonprintable_regex("([[:print:]] ).*") 
    std::smatch matched_message;
    std::regex_search(message, matched_message, trim_nonprintable_regex);

    return matched_message[1].str()
  }

But I am not getting the expected result.

Ex:

If I pass "\r\t\r\t\r\t\ruessw7cr9jhmdiy" it should return empty string.

If I pass "asd\r\tdfvdfv" then it should return asd only.

CodePudding user response：

You need to 1) match at the start of string by adding a ^ at the start and 2) by replacing the ([[:print:]] ).* pattern with just [[:print:]]*:

std::string ExtractPrintableString(const std::string& message) {
    std::regex trim_nonprintable_regex("^[[:print:]]*");
    std::smatch matched_message;
    std::regex_search(message, matched_message, trim_nonprintable_regex);

    return matched_message[0].str();
  }

Note that in this case, there is no capturing group, so you need to return matched_message[0].str().

See the online C demo.

Pattern details:

^ - start of string
[[:print:]]* - zero or more printable chars (that are only matched at the start of string due to ^).

CodePudding user response：

In std::regex trim_nonprintable_regex("([[:print:]] ).*") the means that at least 1 character. So it will never lead to an empty string. Change it to * ant it will work. Also you forgot some ;-s

std::string ExtractPrintableString(const std::string& message) {
    std::regex trim_nonprintable_regex("([[:print:]]*).*");
    std::smatch matched_message;
    std::regex_search(message, matched_message, trim_nonprintable_regex);
    return matched_message[1].str();
}