#include <stdio.h>
#include <string.h>
 
int main ()
{
    char* query = "SELECT ?value? FROM Table";
    char newquery[100];
    char* repl_val = "VariableFieldName";
 
    char* pch;
    int start,end;
    int count;
 
    // get start and end indices...
    pch=strchr(query,'?');
    if (pch!=NULL)
    {
      start = pch-query;
      pch=strchr(pch+1,'?');
      end = pch-query;
    }
    // ...or return
    if(!start || !end)
    {
      return 1;
    }
 
    // copy everything before first '?', concat the replacement value
    strncpy(newquery,query,start);
    strcat(newquery,repl_val);
 
    // concat everything after the second '?', one char at a time
    for(count=end; count<strlen(query); count++)
    {
        strcat(newquery,query[count]);
    }
 
    printf("new query: %s", newquery);
 
    return 0;
}

    char* query = "SELECT %s FROM Table";
    char newquery[100];
    char* repl_val = "VariableFieldName";
    sprintf( newquery, query, repl_value );

"SELECT ?sql_field1?, ?sql_field2? FROM Table WHERE QnAField = "What is the question?" ORDER BY ?sort_field? ASC"
 
// should become:
"SELECT PassedField, AnotherPassedField FROM Table WHERE QnAField = "What is the question?" ORDER BY NameField ASC"

#include <stdio.h>
#include <string.h>
 
int do_sql_placeholders(char* source)
{
  int in_placeholder = 0; // indicate if we are inside a possible placeholder
  int pos = 0;
 
  char placeholder[256]; // to store placeholder name
  int placepos = 0;
 
  for (pos=0; pos < (signed int)strlen(source); pos++)
  {
    if (in_placeholder)
    {
      // if continue placeholder
      if (
          (source[pos] >= '0' && source[pos] <= '9') ||
          (source[pos] >= 'A' && source[pos] <= 'Z') ||
          (source[pos] >= 'a' && source[pos] <= 'z') ||
          source[pos] == '_'
        )
      {
        placeholder[placepos] = source[pos];
        placepos++;
      }
      // else, end place holder
      else
      {
        // valid terminated placeholder, interpolate
        if (source[pos] == '?')
        {
          placeholder[placepos] = '\0';
          /*
            instead of printing this out as a match, i need to call getenv(placeholder)
            and replace the placeholder in source with the output of getenv
          */
          printf("matched placeholder '%s' at position %d\n", placeholder, pos);
        }
        // cleanup
        in_placeholder = 0;
 
        placeholder[0] = '\0';
        placepos=0;
      }
    }
    else
    {
      if (source[pos] == '?') // entering a potential placeholder
      {
        in_placeholder = 1;
        placepos = 0;
      }
    }
  }
  return 0;
}
 
int main()
{
 char query[100] = "SELECT ?sql_field1?, ?sql field2? FROM Table WHERE QnAField = 'What is the question?' ORDER BY ?sort_field? ASC";
 
 do_sql_placeholders(query);
 printf("query: %s\n", query);
 
 return 0;
}

(source[pos] >= '0' && source[pos] <= '9')