Thread: Need regexp help

Since "HELLO" is really an exact match (unlike "H.?LLO"), you might as well just iterate through your line one character at a time. But I presume your wanted substring is actually something else, or you wouldn't be asking this question -- unless you just want to check if HELLO is present.

This is more or less what I've been using:

Code:

#include <regex.h>

struct matchspec {
	int bgn;
	int end;
} rgxp;

char *regexp (char *string, char *patrn) {
	short int i, w=0, len;
	char *word = NULL;
	regex_t rgT;
	regmatch_t match;
	regcomp(&rgT,patrn,REG_EXTENDED);
	if ((regexec(&rgT,string,1,&match,0)) == 0) {
		rgxp.bgn = (int)match.rm_so;
		rgxp.end = (int)match.rm_eo;
		len = rgxp.end-rgxp.bgn;
		word=malloc(len+1);
		for (i=rgxp.bgn; i<rgxp.end; i++) {
			word[w] = string[i];
			w++; }
		word[w]=0; //make sure this string is terminated
	}
	regfree(&rgT);
	return word;
}

regexp(yourline,"H.*O") (for example) will put the character positions of the match into global struct rgxp as rgxp.bgn and rgxp.end. The first character in a line is zero and I believe the "end" value is actually the character after the "O". If there is no match, you get a NULL pointer. If there is a match, you get the content of the match returned (ie. "HEL+O" could return "HELLO").

Good luck. nb. this only returns the first match, but qv. the method in my next post for the next match

What about the input of
<mytag>
"HELLO"
</mytag>

If it's well-formed XML, there's no reason to assume that everything will be neatly on one line for you to process.

(re: Salem) okay! parse an .xml into one line, presuming unix linefeeds:

Code:

char *linefile (char *file) {
	size_t len, mem=0;
	char *cumul, *line = NULL;
	static char err[]="ERROR";
	FILE *FST_mine = fopen(file, "r");
	if (FST_mine == NULL) return line;
	while ((line = linein(FST_mine)) != NULL) {
		len = strlen(line);
		if (mem == 0) { mem = len+1;
			cumul = malloc(mem);
			strcpy(cumul,line);
		}
		else {	mem += len;
			cumul = (char *)realloc(cumul,mem);
			strcat(cumul,line);
		}
	}
	if (fclose(FST_mine) != 0) { puts("fclose fail linefile()");
		return err;}
	return cumul;
}

char *myline=linefile("myfile.xml")
Feed that through the above "regexp" and each time it returns convert all characters in "myline" upto the last rgxp.end into spaces (remember to collect bgn and end somewhere):

Code:

for (i=0; i <rgxp.end; i++) myline[i]=32;

Then maybe free(myline) and you'll end up with an array (or whatever you did to collect the bgns and ends) consisting of the character positions for HELLO in the entire file.

(re: Thanatos) oh!

For an explanation of "linein" see one of my posts on the next page...sorry...

Do you want to know how many bugs that contains?

"Do you want to know how many bugs that contains?"

Please please please! I'm not making ANY claims to expertise, but I have been using (more or less*) this method and had no end of success with it (in a text file search tool that gets used all the time without problems), so if you see something that is not right, fire away.

*eg, normally i error check malloc()

Well, for one thing, your file parameter should probably be constant. ' ' is also a lot more readable than 32.

For another, what does linein return? Is it an malloc()'ed value? (If so, why aren't you freeing it?)

What happens if you read a string that is "ERROR"? Why not return NULL on error? [edit] Like you do when the file couldn't be opened? [/edit]

What happens if the file was opened, but empty -- i.e., nothing is read at all? (You'll return a random value.)

I'm sure this isn't an exhaustive list, what with Salem's post.

> cumul = (char *)realloc(cumul,mem);
Never assign the result of realloc back to the same pointer. If it returns NULL, you've leaked memory.

p = realloc(cumul,mem);
if ( p ) culum = p;
else ....

This gives you the opportunity to do something sensible with the memory you still own.

Oh, and casting realloc as well.

> static char err[]="ERROR";
In addition to what dwks mentioned, how would you free this?
Most of the time, you return a pointer to allocated memory, but this really messes things up.

Also, a few more suggestions:

Code:

		else {	mem += len;
			cumul = (char *)realloc(cumul,mem);
			strcat(cumul,line);
		}

Why make strcat() traverse the line when you already know where the NULL terminator is?

Code:

puts("fclose fail linefile()");

That's what perror() is for. Oh, and errors are often reported in the format "linefile(): fclose failed".

Also, I don't know what "unix linefeeds" has to do with this. Since you open the file in "r" mode, a.k.a. "rt" (text) mode, on any platform you should get newline sequences of plain "\n", assuming the file was in the correct format for that platform in the first place.

Yes, I know, we had a discussion about that already. I guess I shouldn't have mentioned it. I just meant it in contrast to "rb", that's all.

pick pick pick!

Anyway, I appreciate it BUT/AND:

What happens if you read a string that is "ERROR"? Why not return NULL on error? [edit] Like you do when the file couldn't be opened?
Well, failing to open a file and failing to close it aren't the same, are they?

What happens if the file was opened, but empty -- i.e., nothing is read at all? (You'll return a random value.)
Huh...I hadn't tried that...I imagaine declaring cumul as NULL initially would prevent that.

what does linein return? Is it an malloc()'ed value? (If so, why aren't you freeing it?)
There is something I don't quite understand here. "linein" obviously returns a char pointer, so to me it would seem the malloc'n'freeing would/could go on OUTSIDE the function. In practice I just use the pointer, which is local to another function and freed with it I'm told...

Never assign the result of realloc back to the same pointer. If it returns NULL, you've leaked memory.
Huh again. Is that w/r/t realloc in particular, or all functions in general?

Most of the time, you return a pointer to allocated memory, but this really messes things up.
Now we are solidly in "I'm not sure what you mean here" land.

Why make strcat() traverse the line when you already know where the NULL terminator is?
By NULL terminator you mean "\0"? What is my other option (working character by character from mem to mem+len)? In that case, why does strcat even exist?

> static char err[]="ERROR";
In addition to what dwks mentioned, how would you free this?
I wouldn't...

what perror() is for. Oh, and errors are often reported in the format "linefile(): fclose failed".
I'm going straight home to look up perror now. b/t/w you should have seen what it said before I made it "web friendly"...

I don't know what "unix linefeeds" has to do with this.
Sorry, I use linux and am new to C so I hadn't noticed that CR/LF (or whatever) is automatically "/n"

Thanks again.
ps. garcon (if you ever return) despite the issues, it still works like a charm
pps. I like "32" because it's also the number you add to 'A' or 'B' or 'C' to make 'a' and 'b' and 'c'...maybe i should turn this off...