<?php
 
error_reporting (-1);
mb_internal_encoding('utf-8');
 
$text = "г127ггчто-то там ааааа123ррррр 12 у123уу";
$regexp = "/([\s]|^)[а-яёА-ЯЁ][0-9][0-9][0-9][а-яёА-ЯЁ][а-яёА-ЯЁ]([\s]|$)/u";
$matches = array();
 
if (preg_match($regexp, $text, $matches)) {
	echo "Номер есть в тексте: {$matches[0]}\n";
} else {
	echo "Автомобильного номера не обнаружено";
}

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