[整理] python的正则表达式re模块的常用方法

Python 的 re 模块（Regular Expression 正则表达式）提供各种正则表达式的匹配操作，在文本解析、复杂字符串分析和信息提取时是一个非常有用的工具，下面我主要总结了re的常用方法。

原文 : http://www.jb51.net/article/34642.htm#comments/

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re的简介

使用python的re模块，尽管不能满足所有复杂的匹配情况，但足够在绝大多数情况下能够有效地实现对复杂字符串的分析并提取出相关信息。python 会将正则表达式转化为字节码，利用 C 语言的匹配引擎进行深度优先的匹配。

>>> import re 
>>> print re.__doc__
Support for regular expressions (RE).

This module provides regular expression matching operations similar to
those found in Perl.  It supports both 8-bit and Unicode strings; both
the pattern and the strings being processed can contain null bytes and
characters outside the US ASCII range.

Regular expressions can contain both special and ordinary characters.
Most ordinary characters, like "A", "a", or "0", are the simplest
regular expressions; they simply match themselves.  You can
concatenate ordinary characters, so last matches the string 'last'.

The special characters are:
"."      Matches any character except a newline.
"^"      Matches the start of the string.
"$"      Matches the end of the string or just before the newline at
             the end of the string.
"*"      Matches 0 or more (greedy) repetitions of the preceding RE.
             Greedy means that it will match as many repetitions as possible.
"+"      Matches 1 or more (greedy) repetitions of the preceding RE.
"?"      Matches 0 or 1 (greedy) of the preceding RE.
    *?,+?,?? Non-greedy versions of the previous three special characters.
    {m,n}    Matches from m to n repetitions of the preceding RE.
    {m,n}?   Non-greedy version of the above.
"\\"     Either escapes special characters or signals a special sequence.
    []       Indicates a set of characters.
             A "^" as the first character indicates a complementing set.
"|"      A|B, creates an RE that will match either A or B.
    (...)    Matches the RE inside the parentheses.
             The contents can be retrieved or matched later in the string.
    (?iLmsux) Set the I, L, M, S, U, or X flag for the RE (see below).
    (?:...)  Non-grouping version of regular parentheses.
    (?P<name>...) The substring matched by the group is accessible by name.
    (?P=name)     Matches the text matched earlier by the group named name.
    (?#...) A comment; ignored.
    (?=...)  Matches if ... matches next, but doesn't consume the string.
 (?!...) Matches if ... doesn't match next.
    (?<=...) Matches if preceded by ... (must be fixed length).
    (?<!...) Matches if not preceded by ... (must be fixed length).
    (?(id/name)yes|no) Matches yes pattern if the group with id/name matched,
                       the (optional) no pattern otherwise.

The special sequences consist of "\\" and a character from the list
below.  If the ordinary character is not on the list, then the
resulting RE will match the second character.
    \number  Matches the contents of the group of the same number.
    \A       Matches only at the start of the string.
    \Z       Matches only at the end of the string.
    \b       Matches the empty string, but only at the start or end of a word.
    \B       Matches the empty string, but not at the start or end of a word.
    \d       Matches any decimal digit; equivalent to the set [0-9].
    \D       Matches any non-digit character; equivalent to the set [^0-9].
    \s       Matches any whitespace character; equivalent to [ \t\n\r\f\v].
    \S       Matches any non-whitespace character; equiv. to [^ \t\n\r\f\v].
    \w       Matches any alphanumeric character; equivalent to [a-zA-Z0-9_].
             With LOCALE, it will match the set [0-9_] plus characters defined
as letters for the current locale.
    \W       Matches the complement of \w.
    \\       Matches a literal backslash.

This module exports the following functions:
    match    Match a regular expression pattern to the beginning of a string.
    search   Search a string for the presence of a pattern.
    sub      Substitute occurrences of a pattern found in a string.
    subn     Same as sub, but also return the number of substitutions made.
    split    Split a string by the occurrences of a pattern.
    findall  Find all occurrences of a pattern in a string.
    finditer Return an iterator yielding a match object for each match.
    compile  Compile a pattern into a RegexObject.
    purge    Clear the regular expression cache.
    escape   Backslash all non-alphanumerics in a string.

Some of the functions in this module takes flags as optional parameters:
    I  IGNORECASE  Perform case-insensitive matching.
    L  LOCALE      Make \w, \W, \b, \B, dependent on the current locale.
    M  MULTILINE   "^" matches the beginning of lines (after a newline)
as well as the string.
"$" matches the end of lines (before a newline) as well
as the end of the string.
    S  DOTALL      "." matches any character at all, including the newline.
    X  VERBOSE     Ignore whitespace and comments for nicer looking RE's.
 U UNICODE Make \w, \W, \b, \B, dependent on the Unicode locale.

This module also defines an exception 'error'.

可以查询re模块的功能信息，下面会结合几个例子说明。

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re的正则表达式语法

正则表达式语法表如下：

语法	意义	说明
“.”	任意字符
“^”	字符串开始	‘^hello’匹配’helloworld’而不匹配’aaaahellobbb’
“$”	字符串结尾	与上同理
“*”	0 个或多个字符（贪婪匹配）	<*>匹配
“+”	1 个或多个字符（贪婪匹配）	与上同理
“?”	0 个或多个字符（贪婪匹配）	与上同理
*?,+?,??	以上三个取第一个匹配结果（非贪婪匹配）	<*>匹配
{m,n}	对于前一个字符重复m到n次，{m}亦可	a{6}匹配6个a、a{2,4}匹配2到4个a
{m,n}?	对于前一个字符重复m到n次，并取尽可能少	‘aaaaaa’中a{2,4}只会匹配2个
“\”	特殊字符转义或者特殊序列
[]	表示一个字符集	[0-9]、[a-z]、[A-Z]、[^0]
“|”	或	A|B,或运算
(…)	匹配括号中任意表达式
(?#…)	注释，可忽略
(?=…)	Matches if … matches next, but doesn’t consume the string.	‘(?=test)’ 在hellotest中匹配hello
(?!…)	Matches if … doesn’t match next.	‘(?!=test)’ 若hello后面不为test，匹配hello
(?<=…)	Matches if preceded by … (must be fixed length).	‘(?<=hello)test’ 在hellotest中匹配test
(?<!…)	Matches if not preceded by … (must be fixed length).	‘(?<!hello)test’ 在hellotest中不匹配test

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正则表达式特殊序列表如下：

特殊序列符号	意义
\A	只在字符串开始进行匹配
\Z	只在字符串结尾进行匹配
\b	匹配位于开始或结尾的空字符串
\B	匹配不位于开始或结尾的空字符串
\d	相当于[0-9]
\D	相当于[^0-9]
\s	匹配任意空白字符:[\t\n\r\r\v]
\S	匹配任意非空白字符:[^\t\n\r\r\v]
\w	匹配任意数字和字母:[a-zA-Z0-9]
\W	匹配任意非数字和字母:[^a-zA-Z0-9]

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re的主要功能函数

常用的功能函数包括：compile、search、match、split、findall（finditer）、sub（subn）

compile

re.compile(pattern[, flags])

作用：把正则表达式语法转化成正则表达式对象。

flags定义包括：

re.I：忽略大小写

re.L：表示特殊字符集 \w, \W, \b, \B, \s, \S 依赖于当前环境

re.M：多行模式

re.S：’ . ‘并且包括换行符在内的任意字符（注意：’ . ‘不包括换行符）

re.U： 表示特殊字符集 \w, \W, \b, \B, \d, \D, \s, \S 依赖于 Unicode 字符属性数据库

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search

re.search(pattern, string[, flags])
search (string[, pos[, endpos]])

作用：在字符串中查找匹配正则表达式模式的位置，返回 MatchObject 的实例，如果没有找到匹配的位置，则返回 None。

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match

re.match(pattern, string[, flags])
match(string[, pos[, endpos]])

作用：match() 函数只在字符串的开始位置尝试匹配正则表达式，也就是只报告从位置 0 开始的匹配情况，而 search() 函数是扫描整个字符串来查找匹配。如果想要搜索整个字符串来寻找匹配，应当用 search()。

下面是几个例子：

例：最基本的用法，通过re.RegexObject对象调用

#!/usr/bin/env python
import re
r1 = re.compile(r'world')
if r1.match('helloworld'):
print 'match succeeds'
else:
print 'match fails'
if r1.search('helloworld'):
print 'search succeeds'
else:
print 'search fails' 

说明一下：

r是raw(原始)的意思。因为在表示字符串中有一些转义符，如表示回车’\n’。如果要表示\表需要写为’\’。但如果我就是需要表示一个’\’+’n’，不用r方式要写为:’\n’。但使用r方式则为r’\n’这样清晰多了。

例：设置flag

#r2 = re.compile(r'n$', re.S)
#r2 = re.compile('\n$', re.S)
r2 = re.compile('World$', re.I)
if r2.search('helloworld\n'):
print 'search succeeds'
else:
print 'search fails' 

例：直接调用

if re.search(r'abc','helloaaabcdworldn'):
print 'search succeeds'
else:
print 'search fails' 

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split

re.split(pattern, string[, maxsplit=0, flags=0])
split(string[, maxsplit=0])

作用：可以将字符串匹配正则表达式的部分割开并返回一个列表

例：简单分析ip

#!/usr/bin/env python
import re
r1 = re.compile('W+')
print r1.split('192.168.1.1')
print re.split('(W+)', '192.168.1.1')
print re.split('(W+)', '192.168.1.1', 1) 

结果如下：

[‘192’, ‘168’, ‘1’, ‘1’]

[‘192’, ‘.’, ‘168’, ‘.’, ‘1’, ‘.’, ‘1’]

[‘192’, ‘.’, ‘168.1.1’]

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findall

re.findall(pattern, string[, flags])
findall(string[, pos[, endpos]])

作用：在字符串中找到正则表达式所匹配的所有子串，并组成一个列表返回
例：查找[]包括的内容（贪婪和非贪婪查找）

#!/usr/bin/env python
import re
r1 = re.compile('([.*])')
print re.findall(r1, "hello[hi]heldfsdsf[iwonder]lo")
r1 = re.compile('([.*?])')
print re.findall(r1, "hello[hi]heldfsdsf[iwonder]lo")
print re.findall('[0-9]{2}',"fdskfj1323jfkdj")
print re.findall('([0-9][a-z])',"fdskfj1323jfkdj")
print re.findall('(?=www)',"afdsfwwwfkdjfsdfsdwww")
print re.findall('(?<=www)',"afdsfwwwfkdjfsdfsdwww") 

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finditer

re.finditer(pattern, string[, flags])
finditer(string[, pos[, endpos]])

说明：和 findall 类似，在字符串中找到正则表达式所匹配的所有子串，并组成一个迭代器返回。

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sub

re.sub(pattern, repl, string[, count, flags])
sub(repl, string[, count=0])

说明：在字符串 string 中找到匹配正则表达式 pattern 的所有子串，用另一个字符串 repl 进行替换。如果没有找到匹配 pattern 的串，则返回未被修改的 string。Repl 既可以是字符串也可以是一个函数。
例：

#!/usr/bin/env python
import re
p = re.compile('(one|two|three)')
print p.sub('num', 'one word two words three words apple', 2) 

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subn

re.subn(pattern, repl, string[, count, flags])
subn(repl, string[, count=0])

说明：该函数的功能和 sub() 相同，但它还返回新的字符串以及替换的次数。

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