The input is a string. Wherever a * sign occurs in string repeat copies of character on the right hand side of * sign. The number of copies is placed on left hand side of * sign.

input = '5*3';
output = '33333';

If the number of copies has two digits (never three or more digits) it will be presented in square brackets (the right hand side will always be a single character)

input = '[10]*3';
output = '3333333333'

See the test suite for more examples.