Is there a way to validate the syntax of regex pattern using C code

Question

The regex pattern is :

However its not a valid pattern as the syntax is wrong (there should be a . preceding * in 2 places)

Correct pattern is :

Is there a way to validate the syntax of these regex patterns in C programming? Any library or function which would evaluate the above wrong pattern and return invalid pattern? I tried using regcomp (but it did not return invalid pattern for the wrong input)

regex are not a part of the C language, so you would need to find an external library. — Morten Jensen, Feb 01 '19 at 11:21
This would be helpful https://stackoverflow.com/a/3643454/10035556 — Mayur, Feb 01 '19 at 11:22
regcomp did not return invalid pattern for the wrong input which i have provided — Person, Feb 01 '19 at 11:28
which library can be used to serve the purpose of validation? — Person, Feb 01 '19 at 11:28
Please don't include text as images. If anyone wants to copy that, they won't be able to — James Z, Feb 01 '19 at 11:38
actually i pasted the original text, but few characters were not seen correctly after posting it — Person, Feb 01 '19 at 11:40
PIck a regular expression library. Use it to try to compile the expression, see if that succeeds or not. Note that if your RE is being stored as a string literal, you'll have to escape the backslashes in it so that they're treated as literal characters and not the start of a escape sequence themselves.. — Shawn, Feb 01 '19 at 11:45

score 0 · Answer 1 · answered Feb 01 '19 at 11:34

0

You might want to go overboard and do it in C with a library, or check out these live testing tools https://regex101.com/ or https://www.debuggex.com/

answered Feb 01 '19 at 11:34

Gunther Schulz

575
3
18

I do not want to use live testing tools, it would be good if i can do the validation in code itself (C or C++) – Person Feb 01 '19 at 11:37
@Person `C` or `C++` - pick one. `C++` has a `regex` library. – Fureeish Feb 01 '19 at 11:40
C is what i want, if there is no way out in C, then C++ is the second option. – Person Feb 01 '19 at 11:41

score 0 · Answer 2 · answered Feb 01 '19 at 12:10

There are regex libraries for c you can use (see Regular expressions in C: examples? ). When you want to find out, if a given string has valid regex format, you can do this by using another regex ( see Is there a regular expression to detect a valid regular expression?), or you can try to "compile" the string as regex. I think the first way is the cleaner way.

nwellnhof · Answer 3 · 2019-02-01T13:53:27.040

It depends on the regex implementation you're using. Here's an example using POSIX extended regular expressions that simply checks the return value of regcomp and prints an error message obtained with regerror:

#include <regex.h>
#include <stdio.h>
#include <string.h>

void test(const char *regex) {
	regex_t preg;
	int errcode = regcomp(&preg, regex, REG_EXTENDED);
	if (errcode == 0) {
		printf("%s => Success\n", regex);
		regfree(&preg);
	}
	else {
		char errmsg[80];
		regerror(errcode, NULL, errmsg, 80);
		printf("%s => %s\n", regex, errmsg);
	}
}

int main() {
	test("(*\\.com)");
	test("(.*\\.com)");
	return 0;
}

Try it online!

This should print something like:

(*\.com) => Invalid preceding regular expression
(.*\.com) => Success

Note that (*\.com) is a valid POSIX basic regex since an unescaped ( matches a literal (. With basic regexes, a * at the beginning of a regex or parenthesized subexpression also matches a literal *.

With the same code, if we try the above pattern(what i have mentioned in the question) , it fails to throw error, rather compiles successfully (no . before *) — Person, Feb 01 '19 at 16:15

Is there a way to validate the syntax of regex pattern using C code

3 Answers3