Keep in mind there's a third option, the block form of `sub`. Sometimes you need it. Say you want to replace some text with the reverse of that text. You can't use $1 because it's not bound quickly enough:
"foobar".sub(/(.*)/, $1.reverse) # WRONG: either uses a PREVIOUS value of $1,
# or gives an error if $1 is unbound
You also can't use `\1`, because the `sub` method just does a simple text-substitution of `\1` with the appropriate captured text, there's no magic taking place here:
"foobar".sub(/(.*)/, '\1'.reverse) # WRONG: returns '1\'
So if you want to do anything fancy, you should use the block form of `sub` ($1, $2, $`, $' etc. will be available):
"foobar".sub(/.*/){|m| m.reverse} # => returns 'raboof'
"foobar".sub(/(...)(...)/){$1.reverse + $2.reverse} # => returns 'oofrab'