# [R], 75 bytes

<!-- language-all: lang-r -->

    function(s,x=tabulate(utf8ToInt(tolower(s)),122)[97:122])cat(x[!!x],sep="")

[Try it online!][TIO-kx7jsyee]

[R]: https://www.r-project.org/
[TIO-kx7jsyee]: https://tio.run/##dY49C8IwGIRn@yvSKJJIO6QOfkAHN90Fh9LhNU0gEJLSvNHqn6@tm6K33PDccdcNuhx0dBKNdyxkfYlwjRZQsYh6e/Ynhwy99XfVscB5JoqCV7vNfvSaS0DWV2na11lQbUkpHzSjIPXzeaB8PiuESKYMTShPRnJU1npy8Z1t0okLIdbfmTzP0@Vqvpi4cTewpiHGtREzAu5BOhWiRWICASlVO7616mcf3gMfCP6red/diOEF "R – Try It Online"

Prints the result. For invalid input, prints nothing.

Test harness taken from [pajonk's answer](https://codegolf.stackexchange.com/a/238546/67312).