Check if string is a potential match for regex

Question

Basically, I want to have a regular expression, such as # [0-9] +, and be able to check if a string can match it. For example, if I get user input and they enter "#", this line does not match the regular expression, but maybe if the user also entered some numbers.

I know that C ++ has matches () functions, but is there something there, like what I'm looking for? Or some way to do this?

thanks

Answer 1

DISCLAIMER: The following is a rather naive approach that is neither quick nor beautiful. However, it does the job for a simple regular expression. I do not recommend using this without understanding what he is doing.

#include <string> #include <iostream> #include <regex> bool is_valid_regex_string(const std::string& rgx_str) { bool bResult = true; try { std::regex tmp(rgx_str); } catch (const std::regex_error& e) { (e); bResult = false; } return bResult; } std::string create_partial_regex_string(const std::string& rgx_str) { std::string par_rgx_str; for (int i = 1; i <= rgx_str.size(); i++) { std::string sub_rgx_str = rgx_str.substr(0, i); if (is_valid_regex_string(sub_rgx_str)) { if (!par_rgx_str.empty())par_rgx_str += "|"; par_rgx_str += "(" + sub_rgx_str + ")"; } } //return par_rgx_str; return "^" + par_rgx_str + "$"; } void testPartialRegex(const std::string& rgx, const std::string& str) { std::string partialRegexString = create_partial_regex_string(rgx); std::regex partRegex(partialRegexString); std::cout << "\tTESTING \"" << str << "\" against \"" << partialRegexString << "\" :" << std::endl; std::smatch base_match; std::cout << "\t\t-> " << (std::regex_match(str, partRegex) ? "true" : "false") << std::endl; } void test(const std::string& str) { std::cout << "\n###########################################\nTESTING \"" << str << "\"\n" << std::endl; for (int i = 1; i <= str.size(); i++) { testPartialRegex("#[0-9]+", str.substr(0, i)); } std::cout << "\n###########################################\n" << std::endl; } int main() { test("#123456"); test("#12a3456"); test("#b"); test("123456"); test("##"); return 0; } 

Output:

 ########################################### TESTING "#123456" TESTING "#" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true TESTING "#1" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true TESTING "#12" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true TESTING "#123" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true TESTING "#1234" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true TESTING "#12345" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true TESTING "#123456" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true ########################################### ########################################### TESTING "#12a3456" TESTING "#" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true TESTING "#1" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true TESTING "#12" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true TESTING "#12a" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false TESTING "#12a3" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false TESTING "#12a34" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false TESTING "#12a345" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false TESTING "#12a3456" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false ########################################### ########################################### TESTING "#b" TESTING "#" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true TESTING "#b" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false ########################################### ########################################### TESTING "123456" TESTING "1" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false TESTING "12" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false TESTING "123" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false TESTING "1234" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false TESTING "12345" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false TESTING "123456" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false ########################################### ########################################### TESTING "##" TESTING "#" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> true TESTING "##" against "^(#)|(#[0-9])|(#[0-9]+)$" : -> false ########################################### 

Answer 2

You can use Boost.Regex , which already implements Partial matches .

When used, indicates that partial as well as full matches should be found. a partial match is one that matches one or more characters at the end of text input , but does not match all regular expressions (although perhaps more input was available).

the code

 #include <iostream> #include <boost/regex.hpp> using namespace std; using namespace boost; int main () { string subject = "#"; string pattern = "#[0-9]+"; const regex e(pattern); if (regex_match(subject, e, match_partial)) { cout << subject << " \tMATCHES\t " << pattern << endl; } else { cout << subject << " \tDOESN'T MATCH\t " << pattern << endl; } return 0; } 

demo version of rexter

Answer 3

The following code checks to see if the string matches the expression with a "#" at the beginning and then one or more numbers ...

 #include <iostream> #include <regex> using namespace std; int main() { string str; while (true) { cin >> str; regex pat{ "(#[[:d:]]+)" }; bool match = regex_search(str, pat); cout << (match ? "Matched" : "Not matched") << endl; } return 0; }