$re = '/((?:"group1"|\G(?!^))(?:(?!"group2"|"variable").)*"variable":)\d*/m';
$str = '{"mastercont":{"subcont":368,"whatever":236,"anothergroup":{"ItemsG1":["cf8013791a1948e9ad7ec913353cf108","7093f33afa794316af1a6e43d3e3e42f","38670584b898415e9e2e3b2dee39610c","d4d17e6a40ee4b4187a421ac230b6bb0","053c112d38384fa6bff09c01fa48f10d"],"items":[{"id":"14cec419535e449e9d367cb89dfe9b45","Template":"4fdf0640c1094f6cad1d37c332d01180","var3":10,"var2":3035385,"prop":{"group1":[{"id":"e4d9f3206a0d4dd3947a9ab887e2a866","variable":8},{"id":"33f866a5f2f546afac13fa5f02bedfbf","variable":8}]}},{"id":"a5c48bf889cd4a609ef32582c034a137","Template":"023bbfbbd15646ce8eb92cad079142fd","var3":10,"var2":2627635,"prop":{"group1":[{"id":"b4de8d1bd8a145758517c1e5800a7525","variable":5}]}},{"id":"4266dd790972412b8202506f82f07bce","Template":"d501df0303664ee798085626d1b5f58a","var3":10,"var2":2687981,"prop":{"group1":[{"id":"c40ed85187774d09b1690223dae8f67d","variable":7},{"id":"1674a05b355a4c0199947a28e2654d4c","variable":7},{"id":"bf107d7d87774411b07a56819a58a709","variable":7}]}},{"id":"8d9215eb57bd4a9c97c97042602f7385","Template":"0e24274c1f9f4a75b72459237776a6f5","var3":10,"var2":2776081,"prop":{"group1":[{"id":"464bedb7a63143b6a2dff65f089d39da","variable":7}]}},{"id":"d461eb38afc445b4be85c8fd9ca4b61e","Template":"12ab90763c034e628f89f36fbe08bd0c","var3":10,"var2":3991635,"prop":{"group1":[{"id":"848e02b432ae4e1b809c83bca039542a","variable":10}]}}]},"loadout":{"equippedItems":{"48021ab1a1a6487b80a4ca472a4d0c77":{"id":"cf8013791a1948e9ad7ec913353cf108","Template":"4535a4bf490a46d5b49ca9bc0cc81fe6","var3":10,"var2":287004,"prop":{"group1":[{"id":"e4d9f3206a0d4dd3947a9ab887e2a866","variable":10},{"id":"c02cf3392c634456bdd1f4ad1e25bf3f","variable":10},{"id":"e8e0bb2627514cacbd20effdec8dea4d","variable":10}]},"slot":"48021ab1a1a6487b80a4ca472a4d0c77"},"36d141e47783466c95656f90f09de428":{"id":"f68adc253dff47f586ba5229b8ddfe81","Template":"1ca7737c1eef462ab85d922dfb5eb5b6","grade":3,"prop":{"group1":[{"id":"80980773a3884f719ba5ac69a7e307be","variable":9},{"id":"2e56b62015ce4db29eef2faf139f6185","variable":9},{"id":"0717438e41e04414913ca089a60666c0","variable":9}],"group2":[{"id":"ff35b37f7bc3455f81d73a2b60417ab5","variable":1},{"id":"2fc557411c8946cd85e2a8a27f6cea38","variable":1},{"id":"7ed4d49b50534a5dbbacb32a96871a85","variable":1}]},"slot":"36d141e47783466c95656f90f09de428"},"417e79dec81042f88273f9759df6ae25":{"id":"d4d17e6a40ee4b4187a421ac230b6bb0","Template":"9094f0d120c84013ab3877177a009a78","var3":10,"var2":4109,"prop":{"group1":[{"id":"c40ed85187774d09b1690223dae8f67d","variable":7}]},"slot":"417e79dec81042f88273f9759df6ae25"},"897a600c91d64449af09173da88a907e":{"id":"053c112d38384fa6bff09c01fa48f10d","Template":"c74b89cb19cc45299c7eec6b53bcac17","var3":10,"var2":326029,"prop":{"group1":[{"id":"e4d9f3206a0d4dd3947a9ab887e2a866","variable":10}]},"slot":"897a600c91d64449af09173da88a907e"},"862605dec67f4bceb5274e5fb6f25162":{"id":"21e4c1490f0c4c7bbe3b012a51823712","Template":"875aa16e9d2646a0934e6ba5b2a5299f","var3":10,"var2":5044607,"prop":{"group1":[{"id":"b64fa9d44f134bb4912916df08e84757","variable":10},{"id":"a869f868c23f45bb9159d62b42236a4a","variable":10}]},"slot":"862605dec67f4bceb5274e5fb6f25162"},"e273a4d7fb874f7e8f1e398be59afbcb":{"id":"7093f33afa794316af1a6e43d3e3e42f","Template":"b73fa9abfdd049a08b16de3b8a8961fd","var3":10,"var2":287004,"prop":{"group1":[{"id":"e4d9f3206a0d4dd3947a9ab887e2a866","variable":10},{"id":"c02cf3392c634456bdd1f4ad1e25bf3f","variable":10}]},"slot":"e273a4d7fb874f7e8f1e398be59afbcb"},"58b6d1212e234fa4b892c92ae2e2c4c5":{"id":"38670584b898415e9e2e3b2dee39610c","Template":"67b78223916e440b8ec6fc14b83e1150","var3":10,"var2":417904,"prop":{"group1":[{"id":"626121b7708d455fb32f38e5b68bb80c","variable":9}]},"slot":"58b6d1212e234fa4b892c92ae2e2c4c5"},"959c1931bf85458792ec8ecaa58b06d5":{"id":"8019f35c15744377ab99eb0b2ad87ec0","Template":"d408a9123a4f4b2abf4da55b78910f08","grade":5,"prop":{"group1":[{"id":"626121b7708d455fb32f38e5b68bb80c","variable":3},{"id":"8fd72d413e824865b49eb576b8384aa9","variable":3}],"group2":[{"id":"f17be1d91c1e451085143d108b92220f","variable":2},{"id":"0eec28838bc14fdda9398292b545e35d","variable":2},{"id":"d17491225ada4017918d990fc7717481","variable":1}]},"slot":"959c1931bf85458792ec8ecaa58b06d5"},"0d8f2023470141e88bd592381d787456":{"id":"c4fe7d61a70642afb4bd62b28add56cb","Template":"d408a9123a4f4b2abf4da55b78910f08","grade":3,"prop":{"group1":[{"id":"626121b7708d455fb32f38e5b68bb80c","variable":7},{"id":"262ece9bbd654876b6985926ca4422be","variable":7}],"group2":[{"id":"8e9928a847934e15ae2491c41aebad9c","variable":1},{"id":"6d4e443734484a6198e812a7e6850c8c","variable":1},{"id":"6bc195682f764c0e8482dee16629dc5b","variable":1}]},"slot":"0d8f2023470141e88bd592381d787456"}}}}}
';
$subst = "${1}10";
$result = preg_replace($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