The function removes white spaces at the beginning and at the end of the string, replaces nonalphabetical characters by underscores, and transforms all uppercase characters to lowercase ones.
The function removes white spaces at the beginning and at the end of the string, replaces nonalphabetical characters by underscores, and transforms all uppercase characters to lowercase ones.
这个函数去掉字符串中开头和结尾的空格，把符号字符替换成下划线，把大写字母转换成小写字母。