tr does not support posix character class 

I noticed that tr does not support the posix character classes. https://en.wikipedia.org/wiki/Regular_expression#Character_classes

gnu-coreutils:
```ShellSession
$ echo 1.9 | tr -dC '[:digit:]'
19
```

uutils-coreutils:
```ShellSession
$ echo 1.9 | result/bin/tr -dC '[:digit:]'

```