Regular Expression in C ???

C itself doesn't have regular expressions, why don't you just code it.

Example 1 - where single character is to be replaced with another single character....

int replace(char * str1,char old_char, char new_char)
{
char * ptr_temp;
char * ptr_in;
char * ptr_out;

    if((ptr_temp=(char *)malloc(strlen(str1)+1))==NULL)
        return 1;
    ptr_in=str1;
    ptr_out=str2;
    while(*ptr_in!=0x00)
    {
        if(*ptr_in==old_char)
        {
            *ptr_out=new_char;
            ptr_in++;
            ptr_out++;
            continue;
        }
        *ptr_out++=*ptr_in++;
     }
     *ptr_out=0x00;
     strcpy(str1,ptr_temp);
     free(ptr_temp);
     return 0;
}

Example 2 - replacing a character with a string...

int replace(char * str2,char old_char, char * new_str)
{
// This function assumes that str2 is big enough to accomodate the new string!
char * ptr_temp;
char * ptr_in;
char * ptr_out;
int    n_chars_to_replace=0;
int    len_temp;
int    len_new_str;

    // Find out how many characters need to be replaced.
    ptr_in=str2;
    while(*ptr_in!=0x00)
    {
         if(*(ptr_in++)==old_char)
             n_chars_to_replace++;
    }
    if(n_chars_to_replace==0)
         return 0;
    len_new_str=strlen(new_str);
    len=strlen(str2)+1+(n_chars_to_replace*(len_new_str-1));
    if((ptr_temp=(char *)malloc(len))==NULL)
        return 1;
    memset(ptr_temp,0x00,len);
    ptr_in=str1;
    ptr_out=str2;
    while(*ptr_in!=0x00)
    {
        if(*ptr_in==old_char)
        {
            strcat(ptr_out,new_str);
            ptr_in++;
            ptr_out+=len_new_str;
            continue;
        }
        *ptr_out++=*ptr_in++;
     }
     *ptr_out=0x00;
     strcpy(str2,ptr_temp);
     free(ptr_temp);
     return 0;
}

newmang:


In Example 1 - You mean ptr_out = ptr_temp; right? Not ptr_out=str2;

By the way what is 0x00 in the while loop and other places?
thanks, sammani.

sammani

You are right re ptr_out, sorry, metal slip of the tongue. The 0x00 is the NULL character used by C to signify the end of a string.

By the way, on reflection example 1 can be made much simpler. Because you are replacing a single character with another single character it is possible to make the change in place in the original string without making a temp string. The code would be as follows...

void replace(char * str1, char old_char, char new_char)
{
char *ptr;

    ptr-str1;
    while(*ptr!=0x00) // Test each char till we hit the trailing NULL.
    {
        if(*ptr==old_char)
            *ptr=new_char;
        ptr++;
    }
    return;
}

Cheers - Gavin

If you are familiar with strtok or strchr to split the strings on the chars that you want to remove (& and ' respectively) and then, simply substitute the chars which you want.

That way, you can leave the complexity of the searching with built in library functions. Also, this will be more optimized.

WR,
Joy

strtok & strchr is a bit of overkill in the first case where he wants to replace a single character with another, I agree you could use strchr with a moving base pointer for the strchr but my code essentially does what that would do anyway.

Using strtok & strchr in the second case where he wants to replace a single character with a string would introduce the complexity of allocating space to store the substrings created by the strtok operation.

I agree that these functions could be used to do the job, its the beauty of C in that there are many ways to denude a feline, each of which has its good & bad points. My method is simple and shows him what is happening, I have used this method often and I don't notice and performance degradation, I dissasembled the binary generated by my compiler(s) after optimisation to see if I could get any speed improvement with embedded asm code but it looks pretty tight to me.

Joy

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