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assert with regular expression

bluefrog (125)

I am attempting to test a regular expression entered by the user.
The format of the entry must comply with "|number|", for example |12| would pass, whereas |12a| would fail.

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#include <iostream>
#include <cassert>
#include <string>
#include <regex>

int main() {

  std::string s = "";
  std::regex reg_expr("|[[:digit:]]|");

  std::cin >> s;

  std::cout << "\nResult string : " << s;

  assert(regex_match(s,reg_expr));

  std::cout << "\nResult string : " << s;

  return 0;
}


The assertion fails though, and I am unsure why.
Runtime assert failure appears as follows:
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|12|

assert_test_concatenated_str: assert_test_concatenated_str.cpp:15: int main(): Assertion `regex_match(s,reg_expr)' failed.
Result string : |12|Aborted (core dumped)


Is it the regular expression or something else I am doing incorrectly?
Peter87 (11238)
I think you need to use + if you want to allow more than one digit.

 
 
std::regex reg_expr("|[[:digit:]]+|");
bluefrog (125)

still fails though, with + added.

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#include <iostream>
#include <cassert>
#include <string>
#include <regex>

int main() {

  std::string s = "";
  std::regex reg_expr("|[[:digit:]]+|");

  std::cin >> s;

  std::cout << "\nResult string : " << s;

  assert(regex_match(s,reg_expr));

  std::cout << "\nResult string : " << s;

  return 0;
}


failure
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|12|

assert_test_concatenated_str: assert_test_concatenated_str.cpp:15: int main(): Assertion `regex_match(s,reg_expr)' failed.
Result string : |12|Aborted (core dumped)
ne555 (10692)
ah, the bars are part of your input.
| has a special meaning in regex, you need to escape them R"(\|[[:digit:]]+\|)"
bluefrog (125)

It works when I do not use escaped "|" characters, but when the escaped bar is used, it fails.

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#include <iostream>
#include <cassert>
#include <string>
#include <regex>

int main() {

  std::string s = "";

  std::cin >> s;

  std::cout << "\nResult string : " << s;

  // std::regex reg_expr("<[[:digit:]]+:"); // this works
  std::regex reg_expr("\|[[:digit:]]+\|");
  assert(regex_match(s,reg_expr));

  std::cout << "\nResult string : " << s;

  return 0;
}


In addition, I get the following compile warning message :
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c++ -g -std=c++14 -pedantic -Wall -Wpointer-arith -Wwrite-strings -Wcast-qual -Wcast-align -Wformat-security -Wformat-nonliteral -Wmissing-format-attribute -Winline -funsigned-char "assert_test_concatenated_str.cpp" (in directory: /c++/learn_c++)
assert_test_concatenated_str.cpp: In function ‘int main()’:
assert_test_concatenated_str.cpp:15:23: warning: unknown escape sequence: '\|'
   std::regex reg_expr("\|[[:digit:]]+\|");
                       ^
assert_test_concatenated_str.cpp:15:23: warning: unknown escape sequence: '\|'
Compilation finished successfully.


Although it still successfully compiles, the run time assert error appears:
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|12|

assert_test_concatenated_str: assert_test_concatenated_str.cpp:15: int main(): Assertion `regex_match(s,reg_expr)' failed.
Result string : |12|Aborted (core dumped)

Thomas1965 (4571)
Try this:
std::regex reg_expr("\\|\\d+\\|");
bluefrog (125)
thanks, spot on
ne555 (10692)
to avoid backslash hell, use raw strings http://en.cppreference.com/w/cpp/language/string_literal
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