Not matching string of regular expression in C programing - regex

Question

Requirement is,
In a string (plan is the variable), For which does not have the sub string "TT", should return success message in the pattern matching ("Regular expression not having TT" message in the console).
I tried [^(TT)], I tried negation,
I tried ^(?!.*TT).$
#include <stdio.h>
#include "regex.h"
int main()
{
regex_t exps;
int r1 =-1;
int r2= -1;
char *pattern="\\^(\\?\\!.*TT).*\\$";
char *plan="TEST QBSE US 5USD charge sample conv offer";
r1=regcomp(&exps, pattern, REG_EXTENDED | REG_NOSUB);
if (r1 == 0)
{
printf("Regular expression is parsed sucessfully:%s \n",pattern);
}
else
{
printf("Regular expression parsing failed.\n");
}
r2=regexec(&exps, plan, (size_t)0, NULL, 0);
if (r2 == 0)
{
printf("Regular expression not having TT \n");
}
else
{
printf("Regular expression is not matched.\n");
}
regfree(&exps);
return 0;
}
current output:
Regular expression is parsed sucessfully :\^(\?!.TT).\$
Regular expression is not matched.

Answer 1

You may use
char *pattern="^[^T]*(T($|[^T]+))*$";
See the C code demo and the regex demo.
Details
^ - start of string
[^T]* - 0 or more chars other than T
(T($|[^T]+))* - 0 or more repetitions of
T - a T char...
($|[^T]+) - ... followed with the end of string ($) or (|) any 1 or more chars other than T ([^T]+)
$ - end of string.
NOTE: If the regex is any kind of regex pattern that you need to negate the way you describe (validate) you should actually do that easier with the code: use a mere char *pattern="TT"; pattern and if regexec return value is not 0 return true (if (r2 != 0)):
#include <stdio.h>
#include <stdlib.h>
#include <regex.h>
int main (void)
{
regex_t exps;
int r1 =-1;
int r2= -1;
char *pattern="TT"; // <-- The regex is simple
char *plan="TEST QBSE US 5USD charge sample conv offer";
r1=regcomp(&exps, pattern, REG_EXTENDED | REG_NOSUB);
if (r1 == 0)
{
printf("Regular expression is parsed sucessfully:%s \n",pattern);
}
else
{
printf("Regular expression parsing failed.\n");
}
r2=regexec(&exps, plan, (size_t)0, NULL, 0);
if (r2 != 0) // <-- Here goes the negation
{
printf("Regular expression not having TT \n");
}
else
{
printf("Regular expression is not matched.\n");
}
regfree(&exps);
return 0;
}
See the C demo online.