POSIX regular expression cheatsheet
| Alphabetic | [:alpha:] |
| Alphanumeric | [:alnum:] |
| Control characters | [:cntrl:] |
| Hex numbers | [:xdigit:] |
| Lowercase | [:lower:] |
| Nonespace characters | [:graph:] |
| Numbers | [:digit:] |
| Printable | [:print:] |
| Punctuation | [:punct:] |
| Tab or space | [:blank:] |
| Uppercase | [:upper:] |
| White space | [:space:] |