\ba Unicode word boundary
Negative Lookahead(?!всемирный) всемирныйthe literal text всемирный (case sensitive) \w*a Unicode word character, repeated any number of times
мирthe literal text мир (case sensitive) \w*a Unicode word character, repeated any number of times
g modifier: global. Finds all matches instead of stopping after the first
u modifier: unicode. Treats the pattern and subject as UTF-16