foreach (array('- Alpha', 'beta', '. Gamma', "'delta'") as $value) {
    preg_match('~[a-z]~i', $value, $match, PREG_OFFSET_CAPTURE);
    print_r($match);
}

Array
(
    [0] => Array
        (
            [0] => A
            [1] => 2
        )
)
Array
(
    [0] => Array
        (
            [0] => b
            [1] => 0
        )
)
Array
(
    [0] => Array
        (
            [0] => G
            [1] => 2
        )
)
Array
(
    [0] => Array
        (
            [0] => d
            [1] => 1
        )
)

if(i>='a' && i<='z' || i>='A' && i<='Z')