A Simple Wildcard Matching Function

• Download demo project with source - 3.56 KB

Introduction

Simple wild card matching with ? and * is something that we use in our every day work just when opening a command prompt and using DIR and DEL.

But how can this be done correctly in your program?

OK, you can use regular expressions, and I recommend this when you already use a bunch of Boost, tr1 or STL stuff in your code. But sometimes, I like it easy and simple without tons of library code in the background. And because I saw a lot of wrong and in complex queries "wrong/failing" code, I just offer this small algorithm here.

Background

This wildcard matching function works just the same way as you expect and know it from the CMD.EXE DIR command.

Using the Code

The function just takes the string to check as a first argument and the mask with or without any wildcard characters as a second argument.
It returns true if the strings match and false if not. It isn't spectacular.

The characters ? and * are treated as wildcards.
A ? character matches exactly one character and doesn't match an empty string.
A * character matches any sequence of characters and an empty string too.
Other characters are compared caseless. I use CharUpper and convert them to uppercase to perform this task. Feel free to use your own favorite way.

Because * matches any character sequence and an empty string WildcardMatch(_T(""),_T("*")) returns true.

The function itself and the documentation is easy and straight forward:

//////////////////////////////////////////////////////////////////////////
//    WildcardMatch
//        pszString    - Input string to match
//        pszMatch    - Match mask that may contain wildcards like ? and *
//    
//        A ? sign matches any character, except an empty string.
//        A * sign matches any string inclusive an empty string.
//        Characters are compared caseless.

bool WildcardMatch(const TCHAR *pszString, const TCHAR *pszMatch)
{
    // We have a special case where string is empty ("") and the mask is "*".
    // We need to handle this too. So we can't test on !*pszString here.
    // The loop breaks when the match string is exhausted.
    while (*pszMatch)
    {
        // Single wildcard character
        if (*pszMatch==_T('?'))
        {
            // Matches any character except empty string
            if (!*pszString)
                return false;

            // OK next
            ++pszString;
            ++pszMatch;
        }
        else if (*pszMatch==_T('*'))
        {
            // Need to do some tricks.

            // 1. The wildcard * is ignored. 
            //    So just an empty string matches. This is done by recursion.
            //      Because we eat one character from the match string, the
            //      recursion will stop.
            if (WildcardMatch(pszString,pszMatch+1))
                // we have a match and the * replaces no other character
                return true;

            // 2. Chance we eat the next character and try it again, with a
            //    wildcard * match. This is done by recursion. Because we eat
            //      one character from the string, the recursion will stop.
            if (*pszString && WildcardMatch(pszString+1,pszMatch))
                return true;

            // Nothing worked with this wildcard.
            return false;
        }
        else
        {
            // Standard compare of 2 chars. Note that *pszSring might be 0
            // here, but then we never get a match on *pszMask that has always
            // a value while inside this loop.
            if (::CharUpper(MAKEINTRESOURCE(MAKELONG(*pszString++,0)))
		!=::CharUpper(MAKEINTRESOURCE(MAKELONG(*pszMatch++,0))))
                return false;
        }
    }

    // Have a match? Only if both are at the end...
    return !*pszString && !*pszMatch;
}

History

• 28 April, 2011 -- Version 1.0