Some of the character classes to use in regular expressions.
| alpha | Any letter, [A-Za-z] |
| upper | Any uppercase letter, [A-Z] |
| lower | Any lowercase letter, [a-z] |
| digit | Any digit, [0-9] |
| alnum | Any alphanumeric character, [A-Za-z0-9] |
| xdigit | Any hexadecimal digit, [0-9A-Fa-f] |
| space | A tab, new line, vertical tab, form feed, carriage return, or space |
| Any printable character | |
| punct | Any punctuation character: ! ' # S % & ' ( ) * + , - . / : ; < = > ? @ [ / ] ^ _ { | } ~ |
| graph | Any character defined as a printable character except those defined as part of the space character class |
| cntrl | Any character not part of the character classes: [:upper:], [:lower:], [:alpha:], [:digit:], [:punct:], [:graph:], [:print:], [:xdigit:] |