CODEWITHSUNDEEP
cregex
Stack
Stack

Reputation: 111

How to use Regex to verify data input from keyboard is real numbers with C languages?

I try to research for REGEX in C and try to understand but I have trouble with pattern of the string type. In this program I want to verify string input is a number(only digits number, not characters, space, or special characters)

 #include<stdio.h>
 #include <regex.h> 
 void print_result(int return_value){
  if (return_value == 0){
    printf("Pattern found.\n");
  }
  else if (return_value == REG_NOMATCH){
    printf("Pattern not found.\n");
  }
  else{
    printf("An error occured.\n");
  }
}
int main() {
  regex_t regex;
  int return_value;
  int return_value2;
  return_value = regcomp(&regex,"[^a-fA-F_][0-9]+",0);
  return_value = regexec(&regex, "4324", 0, NULL, 0);
  return_value2 = regcomp(&regex,"\d+",0);
  return_value = regexec(&regex, "4324", 0, NULL, 0);
  print_result(return_value);  //not found
  print_result(return_value); //no found
  print_result(return_value2);
  return 0;
}

Can you give me some ideas to verify the input. I want find another way without use ASCII values

Upvotes: 0

Views: 54

Answers (1)

rici
rici

Reputation: 241901

If you specify the flags as 0 in regcomp:

return_value = regcomp(&regex,"[^a-fA-F_][0-9]+",0);

then you are accepting the default regex syntax, which is a so-called Basic Regular Expression (BRE). The only sensible thing that can be said about BREs is "don't use them." Always specify the REG_EXTENDED flag (at least), and then you will be working with a regular expression syntax that at least bears a passing resemblance to what you expect. (Otherwise, your strings will be dominated by what's technically called "leaning timber": \ characters which enable metacharacters in the regex, and more \ characters so that the \ characters you need are not treated as escape characters in the character string.)

Take a look at man regexec and man 7 regex for more details. Make sure you read the second link thoroughly (although you can ignore basic regular expression syntax :-) ) because there are many commonly-used syntaxes in more modern regex libraries which are not present in Posix regexes, not even extended ones. (That includes \d, used in your second regex. Posix has named character classes, such as [[:digit:]].)

Upvotes: 2

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