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 = str.gsub(re, subst)
# Print the result of the substitution
puts 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 Ruby, please visit: http://ruby-doc.org/core-2.2.0/Regexp.html