双引号内的字符串中支持转义字符
Escape Sequence Description
\a Bell (beep)
\b Backspace
\cn The Ctrl+n character
\e Escape
\E Ends the effect of \L, \U or \Q
\f Form feed
\l Forces the next letter into lowercase
\L All following letters are lowercase
\n Newline
\r Carriage return
\Q Do not look for special pattern characters
\t Tab
\u Force next letter into uppercase
\U All following letters are uppercase
\v Vertical tab
\L、\U、\Q功能可以由\E关闭掉，如：
$a = "T\LHIS IS A \ESTRING"; # same as "This is a STRING"
