Question

I want to validate a user input against format specifier.For example format specifiers %4.5f , %4f , %10.5f, etc., If i specify %5.2f as format specifier I should able to validate user input whether its satisfies the given format %5.2f or not?

Note : I am using C++ in windows environment.

Answer 1

You will want to do string matching against a regular expression. The expression should be:

[0-9]{n}\.[0-9]{m}

Here n and m are the number of digits to want. You can build a regular expression from a string, so just make the pattern string from your parameters and then construct the regex; it’s all in <regex> on C++0x compilers (or <tr1/regex> otherwise).

Be careful to either match the entire input string or surround the expression by word boundaries to avoid 12345.67890 matching against m=2, n=2 as “45.67”.

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Edit: Here’s one approach:

std::regex make_pattern(size_t m, size_t n)
{
  std::ostringstream ss;
  ss << "\b([0-9]{" << std::dec << m << "}\\.[0-9]{" << n << "})\b";
  return std::regex(ss.str());
}

/* later */

std::regex rx = make_pattern(4, 5);
std::string input = get_input();
const bool is_valid = std::regex_match(input, rx);