Replace `\d` with `[0-9]` for portable `grep` patterns.
Replace `\d` with `[0-9]` for portable `grep` patterns.

`\d` is only supported in Perl regular expressions and `-P` is not
supported on macOS `grep`. Use explicit `[0-9]` character class to avoid
portability issues.

Closes gh-5

alphabetum/bash-boilerplate