strcspn

Defined in header `<string.h>`
size_t strcspn( const char dest, const char src );

Returns the length of the maximum initial segment of the null-terminated byte string pointed to by dest, that consists of only the characters not found in the null-terminated byte string pointed to by src.

The behavior is undefined if either dest or src is not a pointer to a null-terminated byte string.

Parameters

dest	-	pointer to the null-terminated byte string to be analyzed
src	-	pointer to the null-terminated byte string that contains the characters to search for

Return value

The length of the maximum initial segment that contains only characters not found in the null-terminated byte string pointed to by src

Notes

The function name stands for "complementary span" because the function searches for characters not found in src, that is the complement of src.

Example

Run this code

#include <string.h>
#include <stdio.h>
 
int main(void)
{
    const char *string = "abcde312$#@";
    const char *invalid = "*$#";
 
    size_t valid_len = strcspn(string, invalid);
    if(valid_len != strlen(string))
       printf("'%s' contains invalid chars starting at position %zu\n",
               string, valid_len);
}

Output:

'abcde312$#@' contains invalid chars starting at position 8

References

C11 standard (ISO/IEC 9899:2011):

7.24.5.3 The strcspn function (p: 368)

C99 standard (ISO/IEC 9899:1999):

7.21.5.3 The strcspn function (p: 331)

C89/C90 standard (ISO/IEC 9899:1990):

4.11.5.3 The strcspn function

strspn	returns the length of the maximum initial segment that consists of only the characters found in another byte string (function)
wcscspn (C95)	returns the length of the maximum initial segment that consists of only the wide chars not found in another wide string (function)
strpbrk	finds the first location of any character in one string, in another string (function)

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