What is the simplest way to get a substring in Ruby by removing a specific number of characters from the end?

What is the simplest way to get a substring in Ruby by removing a specific number of characters from the end?

I have the following string:

a_string = "a1wer4zx"

I want to remove the last 3 characters, resulting in "a1wer". Currently, my approach feels overly complicated:

an_array = a_string.split(//, (a_string.length - 2))  
an_array.pop  
new_string = an_array.join  

What is a more efficient way to achieve this using Ruby substring operations?