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JavaScript

var string = 'Right (99.99руб.) OR Left with space (руб. 99.99) OR Right with space (99.99 руб.) OR Left (руб.99.99)';
var re = /руб\.\ ?(\d[\d.]+)|(\d[\d.]*)\ ?руб/g;
string = string.replace(re, '$1$2&&');
alert(string);