$re = '/^.*?([\d.]+\s*(x|х)\s*[\d.]+\s*\/.*ET-?\d+).*$/m';
$str = '1) София 5.5.x13/4x98 ET35
2) Палладика 7x16/5x139.7 D98 ET5 Алмаз черный
3) 35TH Anniversary 8x18/5x108 ET27 Black
4)Уран 6x14/4x108 ET37
1) NG277 5.5x13/4x100 D73.1 ET38
BKF = 5.5x13/4x100 D73.1 ET38
2) MC7/B 7x17/5x114.3 D70.6 ET54
Silver = 7x17/5x114.3 D70.6 ET54
3) S 7 Modify 7.5x18/5x112 D73 ET42
Silver = 7.5x18/5x112 D73 ET42
4) MK-Course Mod.08 7.5x18/5x100 D56.1 ET55
5) Fever-5R 8x18/5x120 D74.1 ET15 Black Mirror
6) R6162 9x20/5x108 D67.1 ET40 S
7) 35 Anniversary 8x18/5x120 ET34 Black
8) Lounge 8 & 10 8x18/5x110 D75 ET38 Black
9) RSL 5068TL 8.5x18/5x112 D71.6 ET40 MLB
10 Килиманджаро-5-оригинал 6x15/5x114.3 D67.1 ET52.5
11) RK L31F 6x15/4x100 D54.1 ET48 S
12 X-113 7x17/5x115 D70.1 ET41 BK/FP
H-323 8x15/6x139.7 H-323 8x15/6x139.7 D110.5 ET-28 HS
D/P H-526 7x15/6x139.7 D110.5 ET-13 HS
LS182 10x15/5x139.7 D108.5 ET-40 BKF
Талисман-Мега 7х16/5x139.7 D95.3 ET35
блэк платинум P0457 8.5 x 20/5x100 D73.1 ET35
Chrome 7100 7x16/6x139.7 D112.2 ET-15 Silver';
$subst = "\1";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php