// re2c $INPUT -o $OUTPUT 
#include <assert.h>

// expect a null-terminated string
static int lex(const char *YYCURSOR)
{
    int count = 0;
loop:
    /*!re2c
    re2c:define:YYCTYPE = char;
    re2c:yyfill:enable = 0;

    *      { return -1; }
    [\x00] { return count; }
    [a-z]+ { ++count; goto loop; }
    [ ]+   { goto loop; }

    */
}

int main()
{
    assert(lex("") == 0);
    assert(lex("one two three") == 3);
    assert(lex("f0ur") == -1);
    return 0;
}