Ideas for implementing wildcard matching in C

Question

I'm looking into writing a program that takes a text, some wildcard patterns and then shows me the words that match those wildcards. The wildcard pattern contains only ., which represents only one character, or *, which can be anything (including a white space, a new line or ?!,./\ etc.).

Until now, I managed to dynamically read the text and I'm thinking into creating a list with all the words with strtok. Only with ., it'd be easy, but I have no idea how to work with *. And expressions, of course, can be a combination of . and *, like this: h.*. (which can be a match for harry for example).

I'd like you to share some ideas with me. I'm not asking for explicit full code, but for ideas to implement it myself.

The character `*` by itself is not a valid regular expression, in a regex it's a modifier character that means "zero or more of the previous expression". It's a valid expression when [*globbing*](https://en.wikipedia.org/wiki/Glob_%28programming%29) on the other hand. — Some programmer dude, Dec 01 '15 at 13:10
In this problem, if an expression would be only *, it should show me all the words. It's a combination of regular expression and globbing, but only with two wildcards. — Adrian Pop, Dec 01 '15 at 13:12
And don't try to make a regular expression engine all by your self, not unless you want to get your hands dirty with finite state machines and automatas and regular languages. Not even a simple globbing system is, well, simple. For globbing, there are functions available, either as part of the C runtime library or as external libraries (depending on system). On POSIX systems (like e.g. Linux or OSX) you could use [`fnmatch`](http://man7.org/linux/man-pages/man3/fnmatch.3.html) for globbing. — Some programmer dude, Dec 01 '15 at 13:16
You can build a simple *wildcard* matching scheme for * and ? in a few dozen lines of C. A *regular expression* search is considerably harder to implement. I'm sure your university task is the former. Check the exact requirements before delving in. — Bathsheba, Dec 01 '15 at 13:22
Okay, for only `.` (or `?`) and `*` it should be a little simpler. But there are still lots of cases that will be hard, like will `"* bar"` match e.g. `"hello foo bar"` or only `"foo bar"`, for example. You could actually find some source for a `scanf` function, and see how it handles pattern matching. — Some programmer dude, Dec 01 '15 at 13:22
I tried to translate what I need in English from my language. Wildcard matching scheme sounds much better :) — Adrian Pop, Dec 01 '15 at 13:30
(Meandering thought only) If `.` stands for 'one single character' and `*` for 'one or more', then your sample sequence `h.*.` can be simplified to just `h*`. Do you want to have suffix functionality as well? I.e., `h*y` – literal characters *after* a wildcard? — Jongware, Dec 01 '15 at 13:47
@Jongware - No, the first requires at least three characters and the second only two. — owacoder, Dec 01 '15 at 13:48
The expressions I have to search for are purely random. Of course, h.*. and h* are equivalent for harry, it was just an example :) Thanks anyone for your help! — Adrian Pop, Dec 01 '15 at 14:00
I wrote an implementation in C# you way wish to see. It has numerous features such as multiple queries on the trot, case insensitive support, and efficiency: https://stackoverflow.com/a/54820605/848344 — Dan W, Feb 22 '19 at 06:12

score 4 · Accepted Answer · answered Dec 01 '15 at 13:17

4

There's a 2001 IOCCC one-liner (schweikh, ahem) that does globbing with * for "zero or more characters" and ? for "exactly one character".

Figuring out how that one works is probably enlightening and gives you plenty of ideas.

answered Dec 01 '15 at 13:17

Jens

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Amazing! I bet you know by heart the C precedence table :-) – Déjà vu Dec 01 '15 at 14:50

score 3 · Answer 2 · edited Jul 01 '21 at 12:21

3

Here is pretty simple implementation:

int WildcardCompare(const char* wild, const char* string) 
{
    const char* cp = NULL, *mp = NULL;
    
    while ((*string) && (*wild != '*')) 
    {
        if ((*wild != *string) && (*wild != '?')) 
        {
            return 0;
        }
        wild++;
        string++;
    }
        
    while (*string) 
    {
        if (*wild == '*') 
        {
            if (!*++wild) 
            {
                return 1;
            }
            mp = wild;
            cp = string+1;
        } 
        else if ((*wild == *string) || (*wild == '?')) 
        {
            wild++;
            string++;
        } 
        else 
        {
            wild = mp;
            string = cp++;
        }
    }
        
    while (*wild == '*') 
    {
        wild++;
    }
    return !*wild;
}

edited Jul 01 '21 at 12:21

Dada

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answered Dec 01 '15 at 13:46

Andrew Komiagin

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Thanks! I'll take a look at this as well :) – Adrian Pop Dec 01 '15 at 13:58
I just tried it and there are some...errors. First of all, I get "error: assignment of read-only variable ‘mp’". If I remove "const", it's ok, but a combination of stars and question-marks are fatal for the program. For example, I tried with string = "lampa" and wild="??*mpa" which should return 1, but I get an Segmentation fault error. I'll try to fix something if I can :) – Adrian Pop Dec 02 '15 at 03:45

score 0 · Answer 3 · answered Dec 01 '15 at 13:26

0

Take a look at flex and bison. They should help you with tokenizing and then parsing the regex. From there it should be fairly easy to perform the matching.

answered Dec 01 '15 at 13:26

doron

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Ideas for implementing wildcard matching in C

3 Answers3