How to sort a vector of character based on the numeric value inside of it using R? [duplicate]

I have a vector of characters, for example,

a = c('s1.bmp','s10.bmp','s11.bmp','s2.bmp','s3.bmp','s4.bmp')

This order is what is automatically generated, but I want to sort it into

b = c('s1.bmp','s2.bmp','s3.bmp','s4.bmp','s10.bmp','s11.bmp')

So that the order of the character vector is based on the ascending order of the numeric value inside of the character vector. Is there convenient way to do this?