You’re pretty close, but the regex wouldn’t catch all email formats, and you don’t need to specify A-Za-z, you can just use the “i” flag to mark the entire expression as case insensitive. There are email format cases that are missed (especially subdomains), but this catches the ones I tested.
$string = file_get_contents("example.txt"); // Load text file contents
// don't need to preassign $matches, it's created dynamically
// this regex handles more email address formats like [email protected], and the i makes it case insensitive
$pattern = '/[a-z0-9_\-\+]+@[a-z0-9\-]+\.([a-z]{2,3})(?:\.[a-z]{2})?/i';
// preg_match_all returns an associative array
preg_match_all($pattern, $string, $matches);
// the data you want is in $matches[0], dump it with var_export() to see it
var_export($matches[0]);
output:
array (
0 => '[email protected]',
1 => '[email protected]',
2 => '[email protected]',
3 => '[email protected]',
4 => '[email protected]',
)