$re = '/(.*?)\[(.*)\]/';
$str = 'Canada [CA]
United States [US]
Afghanistan [AF]
Albania [AL]
Algeria [DZ]
American Samoa [AS]
Andorra [AD]
Angola [AO]
Anguilla [AI]
Antarctica [AQ]
Antigua And Barbuda [AG]
Argentina [AR]
Armenia [AM]
Aruba [AW]
Australia [AU]
Austria [AT]
Azerbaijan [AZ]
Bahamas [BS]
Bahrain [BH]
Bangladesh [BD]
Barbados [BB]
Belarus [BY]
Belgium [BE]
Belize [BZ]
Benin [BJ]
Bermuda [BM]
Bhutan [BT]
Bolivia [BO]
Bosnia And Herzegovina [BA]
Botswana [BW]
Bouvet Island [BV]
Brazil [BR]
British Indian Ocean Territory [IO]
Brunei Darussalam [BN]
Bulgaria [BG]
Burkina Faso [BF]
Burundi [BI]
Cambodia [KH]
Cameroon [CM]
Cape Verde [CV]
Cayman Islands [KY]
Central African Republic [CF]
Chad [TD]
Chile [CL]
China [CN]
Christmas Island [CX]
Cocos (keeling) Islands [CC]
Colombia [CO]
Comoros [KM]
Congo [CG]
Congo, The Democratic Republic Of The [CD]
Cook Islands [CK]
Costa Rica [CR]
Cote D\'ivoire [CI]
Croatia [HR]
Cuba [CU]
Cyprus [CY]
Czech Republic [CZ]
Denmark [DK]
Djibouti [DJ]
Dominica [DM]
Dominican Republic [DO]
East Timor [TP]
Ecuador [EC]
Egypt [EG]
El Salvador [SV]
Equatorial Guinea [GQ]
Eritrea [ER]
Estonia [EE]
Ethiopia [ET]
Falkland Islands (malvinas) [FK]
Faroe Islands [FO]
Fiji [FJ]
Finland [FI]
France [FR]
French Guiana [GF]
French Polynesia [PF]
French Southern Territories [TF]
Gabon [GA]
Gambia [GM]
Georgia [GE]
Germany [DE]
Ghana [GH]
Gibraltar [GI]
Greece [GR]
Greenland [GL]
Grenada [GD]
Guadeloupe [GP]
Guam [GU]
Guatemala [GT]
Guinea [GN]
Guinea-bissau [GW]
Guyana [GY]
Haiti [HT]
Heard Island And Mcdonald Islands [HM]
Holy See (vatican City State) [VA]
Honduras [HN]
Hong Kong [HK]
Hungary [HU]
Iceland [IS]
India [IN]
Indonesia [ID]
Iran, Islamic Republic Of [IR]
Iraq [IQ]
Ireland [IE]
Israel [IL]
Italy [IT]
Jamaica [JM]
Japan [JP]
Jordan [JO]
Kazakstan [KZ]
Kenya [KE]
Kiribati [KI]
Korea, Democratic People\'s Republic Of [KP]
Korea, Republic Of [KR]
Kosovo [KV]
Kuwait [KW]
Kyrgyzstan [KG]
Lao People\'s Democratic Republic [LA]
Latvia [LV]
Lebanon [LB]
Lesotho [LS]
Liberia [LR]
Libyan Arab Jamahiriya [LY]
Liechtenstein [LI]
Lithuania [LT]
Luxembourg [LU]
Macau [MO]
Macedonia, The Former Yugoslav Republic Of [MK]
Madagascar [MG]
Malawi [MW]
Malaysia [MY]
Maldives [MV]
Mali [ML]
Malta [MT]
Marshall Islands [MH]
Martinique [MQ]
Mauritania [MR]
Mauritius [MU]
Mayotte [YT]
Mexico [MX]
Micronesia, Federated States Of [FM]
Moldova, Republic Of [MD]
Monaco [MC]
Mongolia [MN]
Montenegro [ME]
Montserrat [MS]
Morocco [MA]
Mozambique [MZ]
Myanmar [MM]
Namibia [NA]
Nauru [NR]
Nepal [NP]
Netherlands [NL]
Netherlands Antilles [AN]
New Caledonia [NC]
New Zealand [NZ]
Nicaragua [NI]
Niger [NE]
Nigeria [NG]
Niue [NU]
Norfolk Island [NF]
Northern Mariana Islands [MP]
Norway [NO]
Oman [OM]
Pakistan [PK]
Palau [PW]
Palestinian Territory, Occupied [PS]
Panama [PA]
Papua New Guinea [PG]
Paraguay [PY]
Peru [PE]
Philippines [PH]
Pitcairn [PN]
Poland [PL]
Portugal [PT]
Puerto Rico [PR]
Qatar [QA]
Reunion [RE]
Romania [RO]
Russian Federation [RU]
Rwanda [RW]
Saint Helena [SH]
Saint Kitts And Nevis [KN]
Saint Lucia [LC]
Saint Pierre And Miquelon [PM]
Saint Vincent And The Grenadines [VC]
Samoa [WS]
San Marino [SM]
Sao Tome And Principe [ST]
Saudi Arabia [SA]
Senegal [SN]
Serbia [RS]
Seychelles [SC]
Sierra Leone [SL]
Singapore [SG]
Slovakia [SK]
Slovenia [SI]
Solomon Islands [SB]
Somalia [SO]
South Africa [ZA]
South Georgia And The South Sandwich Islands [GS]
Spain [ES]
Sri Lanka [LK]
Sudan [SD]
Suriname [SR]
Svalbard And Jan Mayen [SJ]
Swaziland [SZ]
Sweden [SE]
Switzerland [CH]
Syrian Arab Republic [SY]
Taiwan, Province Of China [TW]
Tajikistan [TJ]
Tanzania, United Republic Of [TZ]
Thailand [TH]
Togo [TG]
Tokelau [TK]
Tonga [TO]
Trinidad And Tobago [TT]
Tunisia [TN]
Turkey [TR]
Turkmenistan [TM]
Turks And Caicos Islands [TC]
Tuvalu [TV]
Uganda [UG]
Ukraine [UA]
United Arab Emirates [AE]
United Kingdom [GB]
United States Minor Outlying Islands [UM]
Uruguay [UY]
Uzbekistan [UZ]
Vanuatu [VU]
Venezuela [VE]
Viet Nam [VN]
Virgin Islands, British [VG]
Virgin Islands, U.s. [VI]
Wallis And Futuna [WF]
Western Sahara [EH]
Yemen [YE]
Zambia [ZM]
Zimbabwe [ZW]
Other
';
$subst = "$2|$1[$2]";
$result = preg_replace($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