Returns the length of the prefix of a string that consists
    entirely of characters from a set of characters.

    Format

      #include  <string.h>

      size_t strspn  (const char *str, const char *charset);

1  –  Arguments

 str

    A pointer to a character string. If this string is a null string,
    0 is returned.

 charset

    A pointer to a character string containing the characters for
    which the function will search.

2  –  Description

    The strspn function scans the characters in the string, stops
    when it encounters a character not found in charset, and returns
    the length of the string's initial segment formed by characters
    found in charset.

3  –  Return Value

    x                  The length of the segment.