Character classes

The tr command can use different character classes as sets. Here are the supported character classes:

• alnum: Alphanumeric characters
• alpha: Alphabetic characters
• cntrl: Control (nonprinting) characters
• digit: Numeric characters
• graph: Graphic characters
• lower: Lowercase alphabetic characters
• print: Printable characters
• punct: Punctuation characters
• space: Whitespace characters
• upper: Uppercase characters
• xdigit: Hexadecimal characters

We can select the required classes, like this:

tr [:class:] [:class:]

Consider this example:

tr '[:lower:]' '[:upper:]'