Question

(Update: this question’s main focus is to test the “nothing else” part)

Given a string s, which can contain anything, what is the most correct regexp in Ruby to check whether it is a single digit and nothing else? (a single digit and only a single digit).

Answer 1

Use /\A\d\z/

irb(main):001:0> "asd\n7\n" =~ /\A\d\Z/
=> nil # works as false
irb(main):002:0> "asd\n7\n" =~ /\A\d\z/
=> nil # works as false
irb(main):083:0> "7\n"=~/\A\d\Z/
=> 0 # \Z fails, need \z
irb(main):084:0> "7\n"=~/\A\d\z/
=> nil # works as false
irb(main):005:0> "7" =~ /\A\d\Z/
=> 0 # works as true
irb(main):006:0> "7" =~ /\A\d\z/
=> 0 # works as true

http://www.zenspider.com/Languages/Ruby/QuickRef.html :

\z            end of a string
\Z            end of a string, or before newline at the end