1. To get a literal '%', insert "%%". 如果要得到一个'%'，插入“%%”。
2. All characters are ordinary, the expression is a literal string. 所有字符都是普通的，表达式是字面字符串。
3. A backslash followed by the $ character results in a $ character in the text literal, without doing any variable substitution. 一个反斜杠后跟着一个$字符结果是一个文本字面值的$字符，不做任何变量替换。
英英解释
n.
1. a mistake in printed matter resulting from mechanical failures of some kind
2. being or reflecting the essential or genuine character of something
3. without interpretation or embellishment
4. limited to the explicit meaning of a word or text
5. avoiding embellishment or exaggeration (used for emphasis)
