How do I create a case insensitive regex to match file extensions?

Question

I am trying to match all files that have the extension .nef - The match must be case insensitive.

regex e("(.*)(\\.NEF)",ECMAScript|icase);
...
if (regex_match ( fn1, e )){
    //Do Something
}

here fn1 is a string with a file name.

However, this "does something" only with files with .NEF (upper case) extensions. .nef extensions are ignored.

I also tried -

regex e("(.*)(\\.[Nn][Ee][Ff])");

and

regex e("(.*)(\\.[N|n][E|e][F|f])");

both of which resulted in a runtime error.

terminate called after throwing an instance of 'std::regex_error'
  what():  regex_error
Aborted (core dumped)

My code is compiled using -

g++ nefread.cpp -o nefread -lraw_r -lpthread -pthread -std=c++11 -O3

What am I doing wrong? This is my basic code. I want to extend it to match more file extensions .nef, .raw, .cr2 etc.

That exception probably doesn't have anything do with your regex string, but it's because you're using gcc's regex implementation, which is broken. gcc will ship with a working `` in version 4.9 — Praetorian, Jan 28 '14 at 04:11
Why do you try to match "." with "\\."? Shouldn't it be "\."? — Maria Ines Parnisari, Jan 28 '14 at 04:12
@Praetorian - Oh! So some of these http://cpprocks.com/wp-content/uploads/c++11-regex-cheatsheet.pdf will not work?? — Lord Loh., Jan 28 '14 at 04:13
@l19 - The first ` \ ` (c++) is to escape the second ` \ ` (regex) which is to escape the ` . `. — Lord Loh., Jan 28 '14 at 04:14
[`^(.*)(\.)([Nn][Ee][Ff])$`](http://rubular.com/r/XxHenYdwwS) seems to work correctly. though you should escape the `'\'` as you already know, and includes the extension as a separate capture group (which can be convenient). If this doesn't work on your end you're regex has issues. What regex are you using anyway? — WhozCraig, Jan 28 '14 at 04:18
@WhozCraig - I am using the standard regex (http://www.cplusplus.com/reference/regex/) and that expression did not match any file :-( — Lord Loh., Jan 28 '14 at 04:42

score 5 · Answer 1 · edited May 23 '17 at 12:31

Your original expression is correct, and should produce the desired result. The problem is with the gcc implementation of <regex>, which is broken. This answer explains the historical reasons why it so, and also says that gcc4.9 will ship with a working <regex> implementation.

Your code works using Boost.Regex

#include <iostream>
#include <string>
#include <boost/regex.hpp>

int main()
{
    // Simple regular expression matching
    boost::regex expr(R"((.*)\.(nef))", boost::regex_constants::ECMAScript |
                                        boost::regex_constants::icase);
    //                ^^^           ^^
    // no need escape the '\' if you use raw string literals
    boost::cmatch m;

    for (auto const& fname : {"foo.nef", "bar.NeF", "baz.NEF"}) {
        if(boost::regex_match(fname, m, expr)) {
            std::cout << "matched: " << m[0] << '\n';
            std::cout << "         " << m[1] << '\n';
            std::cout << "         " << m[2] << '\n';
        }
    }
}

Live demo

Thank you. I think I shall use the boost implementation the next time I code. For now, I have used `regex e("(.*)(\\.)(((N|n)(E|e)(F|f))|((C|c)(R|r)2))",ECMAScript|icase);` - the `icase` is useless. to match `.nef` and `.cr2` the way I want it. — Lord Loh., Jan 28 '14 at 04:42
That should work, but you really don't need that horrendous looking expression if you use the `icase` flag. `regex expr(R"dlm((.*)\.(nef|cr2))dlm", ECMAScript|icase);` will get you the same result. — Praetorian, Jan 28 '14 at 04:45

How do I create a case insensitive regex to match file extensions?

1 Answers1