[:alnum:]'
Alphanumeric characters: `[:alpha:]' and `[:digit:]'.
`[:alpha:]'
Alphabetic characters: `[:lower:]' and `[:upper:]'.
`[:blank:]'
Blank characters: space and tab.
`[:cntrl:]'
Control characters. In ASCII,these characters have octal codes
000 through 037,and 177 (`DEL'). In other character sets,these
are the equivalent characters,if any.
`[:digit:]'
Digits: `0 1 2 3 4 5 6 7 8 9'.
`[:graph:]'
Graphical characters: `[:alnum:]' and `[:punct:]'.
`[:lower:]'
Lower-case letters: `a b c d e f g h i j k l m n o p q r s t u v w
x y z'.
`[:print:]'
Printable characters: `[:alnum:]',`[:punct:]',and space.
`[:punct:]'
Punctuation characters: `! " # $ % & ' ( ) * +,- . / : ; < = > ?
@ [ \ ] ^ _ ` { | } ~'.
`[:space:]'
Space characters: tab,newline,vertical tab,form Feed,carriage
return,and space.
`[:upper:]'
Upper-case letters: `A B C D E F G H I J K L M N O P Q R S T U V W
X Y Z'.
`[:xdigit:]'
Hexadecimal digits: `0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f'.
For example,`[[:alnum:]]' means `[0-9A-Za-z]',except the latter depends upon the `C' locale and the ASCII character encoding,whereas the former is independent of locale and character set. (Note that the brackets in these class names are part of the symbolic names,and must
be included in addition to the brackets delimiting the bracket expression.)
[$]方括号内包含的是一个匹配字符范围，前面加上^,即是代表不匹配指定的字符范围。
$ereg代表的是一个变量，即$是变量的标志，这个式子的整体意思就是：
ereg代表的是一个具有以字母开头第二个是字母或数字的所有字符串。