Question

I need my C program to be able to read in an identifier using the scanf() method in C.

An identifier in this case is a letter or a _ character followed by one or more alphanumeric characters including the _ character.

The regular expression would be

    [a-ZA-Z_][a-zA-Z0-9_]*

These are examples of correct identifiers:

    _identifier1
    variable21

These are examples of incorrect identifiers

    12var
    %foobar

Does anybody know how this would be done using scanf() in C?

Answer 1

scanf() doesn’t support regular expressions. There’s no regular expression support at all in the standard C library. You’ll have to read a string and then parse it “manually”.

For example:

#include <stdio.h>
#include <ctype.h>

int isIdentifier(const char* s)
{
  const char* p = s;

  if (!(*p == '_' || isalpha(*p)))
  {
    return 0;
  }

  for (p++; *p != '\0'; p++)
  {
    if (!(*p == '_' || isalnum(*p)))
    {
      return 0;
    }
  }

  return 1;
}

int main(void)
{
  const char* const testData[] =
  {
    "a",
    "a_",
    "_a",
    "3",
    "3a",
    "3_",
    "_3"
  };
  int i;

  for (i = 0; i < sizeof(testData) / sizeof(testData[0]); i++)
  {
    printf("\"%s\" is %san identifier\n",
           testData[i],
           isIdentifier(testData[i]) ? "" : "not ");
  }

  return 0;
}

Output:

"a" is an identifier
"a_" is an identifier
"_a" is an identifier
"3" is not an identifier
"3a" is not an identifier
"3_" is not an identifier
"_3" is an identifier