- preg_replace: documentation ( source)
<?php
$strings = [
"1 800-555-1111",
"1 222-555-1111",
"888-555-1111",
"1 888 555 1111",
"1 ",
"1 2"
];
$pattern = '/^1\h+(?=\d(?:[^\d\n\r]?\d){9}$)/';
foreach ($strings as $s) {
echo preg_replace($pattern, '', $s) . PHP_EOL;
}