Tweak check for presence of letters

Faster, maybe simpler?

src/Faker/Provider/Internet.php

@@ -96,7 +96,7 @@ public function userName()
         $username = strtolower(static::transliterate($username));
 
         // check if transliterate() didn't support the language and removed all letters
-        if (preg_match('/^[._]*$/', $username)) {
+        if (trim($username, '._') === '') {
             throw new \Exception('userName failed with the selected locale. Try a different locale or activate the "intl" PHP extension.');
         }
 
@@ -134,7 +134,7 @@ public function domainWord()
         $lastName = strtolower(static::transliterate($lastName));
 
         // check if transliterate() didn't support the language and removed all letters
-        if (preg_match('/^[._]*$/', $lastName)) {
+        if (trim($lastName, '._') === '') {
             throw new \Exception('domainWord failed with the selected locale. Try a different locale or activate the "intl" PHP extension.');
         }