const regex = new RegExp('<a\\s[^>]*?href\\s*=\\s*\\\\\\'https?://site\\.ru/[^>]+>\\s*<img.+?</a>', 'gs')
const str = `<a class=\\'fancybox\\' href=\\'http://site.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\' rel=\\'group\\'><img src=\\'http://site.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\' width=\\'400px\\'></img></a>
<a class=\\'fancybox\\' href=\\'http://site2.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\' rel=\\'group\\'><img src=\\'http://site2.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\' width=\\'400px\\'></img></a>
<a class=\\'fancybox\\' href=\\'http://site.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\' rel=\\'group\\'><img src=\\'http://site.ru/upload/manuals/3V/image_2022_01_28T12_03_17_757Z.jpg\\' width=\\'400px\\'></img></a>`;
const subst = ``;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', 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 JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions