正则表达式中的一些坑

0.字符组

在Python中使用的是中括号“[]”，其中的元素组成需要是单个的“字符”，就算是用“()”括起来的，也无法表示单个的字符，很容易出错！

测试数据：

"nt":"3gnet"
"nt":"2g/3g"
"nt":"cmnet"
"nt":"cmwap"
"nt":"ctnet"
"nt":"ctwap"

正则表达式：

regex_net = re.compile(r'(3gnet)|(2g/3g)|(c(m|t)(net|wap))', re.I)              #正确！
regex_net = re.compile(r'(3gnet)|(2g/3g)|(c[mt]((net)|(wap)))', re.I)   #正确！
#regex_net = re.compile(r'(3gnet)|(2g/3g)|(c[mt][net|wap])', re.I)             #错误！
#regex_net = re.compile(r'(3gnet)|(2g/3g)|(c[mt][(net)|(wap)])', re.I) #错误！
#regex_net = re.compile(r'(3gnet)|(2g/3g)|(cmnet)|(cmwap)|(ctnet)', re.I)  #太丑陋……

参考链接：

• What is the difference between square brackets and parentheses in a regex? – Stack Overflow
• http://www.regular-expressions.info/brackets.html
• http://www.regular-expressions.info/posixbrackets.html
• 正则表达式中的小括号和中括号，有什么区别呢？_百度知道

1.使用非贪婪匹配表达式时要非常注意的点！

sqli_regex1 = re.compile(r'(select|insert|update|delete).+?(where|values|set)(.+?) </', re.I|re.S)
sqli_regex2 = re.compile(r'(select|insert|update|delete).+?(where|values|set)(.+?)', re.I|re.S)
sqli_regex3 = re.compile(r'(select|insert|update|delete).+?(where|values|set)(.+?)$', re.I|re.S)

第一种sqli_regex1是准确指定，没有问题；
第二种sqli_regex2就存在问题了，此处的非贪婪匹配只能匹配一个字符，而不是匹配至行尾（如果要修改的话，可以将“(.+?)”修改为“(.*?)”即可）；
第三种sqli_regex3也没问题，因为手动指定了“$”行尾标识。

2.一些选项（多行匹配、忽略大小写……）

re.I
re.IGNORECASE
Perform case-insensitive matching; expressions like [A-Z] will match lowercase letters, too. This is not affected by the current locale.

re.L
re.LOCALE
Make w, W, b, B, s and S dependent on the current locale.

re.M
re.MULTILINE
When specified, the pattern character '^' matches at the beginning of the string and at the beginning of each line (immediately following each newline); and the pattern character '$' matches at the end of the string and at the end of each line (immediately preceding each newline). By default, '^' matches only at the beginning of the string, and '$' only at the end of the string and immediately before the newline (if any) at the end of the string.

re.S
re.DOTALL
Make the '.' special character match any character at all, including a newline; without this flag, '.' will match anything except a newline.（ re.S 即为 '.' 并且包括换行符在内的任意字符 ）

re.U
re.UNICODE
Make w, W, b, B, d, D, s and S dependent on the Unicode character properties database.（New in version 2.0.）

3.将多行字符串转换成单行字符串（去掉行内连续的多个空白字符——换行符、空格、Tab符号）

x = string.join(multi_line_str.split(), ‘ ‘)
即，先str.split，然后再string.join()（注意第二个是string.join()而不是str.join()）
https://docs.python.org/2/library/stdtypes.html#str.split
https://docs.python.org/2/library/string.html#string.join

参考链接：

• https://docs.python.org/2/library/re.html#re.finditer
• https://docs.python.org/2/library/re.html
• Python正则表达式指南
• http://www.ibm.com/developerworks/cn/opensource/os-cn-pythonre/index.html