jfhufl

    ruby -e 'x = "a\n25\n" ; if x =~ /^\d+$/ ; puts "yes" ; else ; puts "no" ; end'
    yes

    $ ruby -e 'x = "25" ; if x =~ /^\d+$/ ; puts "yes" ; else ; puts "no" ; end'
    yes
    $ ruby -e 'x = "25\n" ; if x =~ /^\d+$/ ; puts "yes" ; else ; puts "no" ; end' 
    yes
    $ ruby -e 'x = "a25\n" ; if x =~ /^\d+$/ ; puts "yes" ; else ; puts "no" ; end'
    no

    https://en.wikipedia.org/wiki/Number_form
    https://www.discovermagazine.com/health/the-rare-humans-who-see-time-and-have-amazing-memories