如何检查字符串上有一个单词混合字母？

Question

How to check string on it has mixed (latin & cyrillic) symbols in one word? For example:

$str = 'This is test string'; //ok
$str = 'This is тест string'; //ok
$str = 'This is тестstring'; // <-- fail, how to detect this?

More examples:

$str = 'This is тест_123 string'; //ok
$str = 'This is {тест}_string'; //fail
$str = 'Абвгabcd'; //fail
$str = 'Абвг_abcd'; //fail
$str = 'Абвг abcd'; //ok
$str = 'This sentence has русское word'; //ok
$str = 'This has splittedкириллицаletters word'; //fail

Answer 1

Found solution, it passed all tests

$result = preg_match_all('/\S*[а-яА-Я]\S*[a-zA-Z]\S*|\S*[a-zA-Z]\S*[а-яА-Я]\S*/', $str, $matches);