How to strip the digit from a string

I am trying to extract a number from a string using regex for javascript.

Test cases

value[0]        ---> 0
value[1]        ---> 1
object2value[0] ---> 0
object90        ---> no match

i tried using ([w]+)*?[d] but it give

value[0]        ---> value[0]
value[1]        ---> value[1]
object2value[0] ---> object2value[0]
object90        ---> no match

how i can achieve this.