regex ruby valid for letters-digits and digits
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I have 2 regexps in ruby. The first regex is for letters and digits something like:

    letter_digits = ^([0-9]*[a-zA-Z][a-zA-Z0-9]*)$/

With this regex if I write only digits `564521` does not match!

However I have this regex for only digits:

    only_digits = /^([0-9])*$/

But with this regex if I write `s45xo` does not match!

I need only **1 regex valid** for both **letters/digits** and **digits**.