strstr() function like, that ignores upper or lower case

Question

I have two strings. Lets say `

str1="One Two Three";

and

str2="two";

I would like to know if there is any function that checks for a match of the second string in the first one,and returns me a pointer to the first occurrence, something like strstr(), but which doesn't treat the same letter, upper or lowercase, as two different characters.

For my example, the function should find a match for str2 in the first string, despite the uppercase "T", of "Two".

Answer 1

From the manpage for strstr:

STRSTR(3)           Linux Programmer's Manual           STRSTR(3)

NAME
       strstr, strcasestr - locate a substring

SYNOPSIS
       #include <string.h>

       char *strstr(const char *haystack, const char *needle);

       #define _GNU_SOURCE

       #include <string.h>

       char *<b><u>strcasestr</u></b>(const char *haystack, const char *needle);

DESCRIPTION
       The  strstr()  function  finds the first occurrence of the substring needle in
       the string haystack.  The terminating '\0' characters are not compared.

       <b>The strcasestr() function is like strstr(3), but  ignores  the  case  of  both
       arguments.</b>

RETURN VALUE
       These functions return a pointer to the beginning of the substring, or NULL if
       the substring is not found.

So what you're looking for is strcasestr.

Answer 2

While some compiler's C libraries include extensions with case insensitive versions of the standard string functions, such as GNU's strcasestr(), the naming of such functions is not standardised even when included.

One way of overcoming the lack of a standard implementation is of course to implement your own:

char* stristr( const char* str1, const char* str2 )
{
    const char* p1 = str1 ;
    const char* p2 = str2 ;
    const char* r = *p2 == 0 ? str1 : 0 ;

    while( *p1 != 0 && *p2 != 0 )
    {
        if( tolower( (unsigned char)*p1 ) == tolower( (unsigned char)*p2 ) )
        {
            if( r == 0 )
            {
                r = p1 ;
            }

            p2++ ;
        }
        else
        {
            p2 = str2 ;
            if( r != 0 )
            {
                p1 = r + 1 ;
            }

            if( tolower( (unsigned char)*p1 ) == tolower( (unsigned char)*p2 ) )
            {
                r = p1 ;
                p2++ ;
            }
            else
            {
                r = 0 ;
            }
        }

        p1++ ;
    }

    return *p2 == 0 ? (char*)r : 0 ;
}

The test code below outputs:

Two Three
Two Three
NULL
cdefg
CDEFG
CdEfG
NULL
zzzz
NULL

zzzzz
NULL

---

int main(void) 
{
    char* test = stristr( "One TTwo Three", "two" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr( "One Two Three", "two" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr( "One wot Three", "two" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr( "abcdefg", "cde" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr( "ABCDEFG", "cde" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr( "AbCdEfG", "cde" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr( "1234567", "cde" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr( "zzzz", "zz" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr( "zz", "zzzzz" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr( "", "" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr( "zzzzz", "" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr( "", "zzzz" ) ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    test = stristr("AAABCDX","AABC") ;
    printf( "%s\n", test == 0 ? "NULL" : test  ) ;

    return 0;
}