Python: 正则表达式实例透析
re模块
import re
re.search
经常用match = re.search(pat, str)的形式。因为有可能匹配不到，所以re.search()后面一般用if statement。
str = 'an example word:cat!!'
match = re.search(r'word:\w\w\w', str)
if match:
print 'found', match.group() ## 'found word:cat'
else:
print 'did not find'
found word:cat
# re.search return a match object, which contains lots of info
print type(match)
<type '_sre.SRE_Match'>
print match.string # source string
print match.group()
print match.start() # position of w
print match.end() # position of t
print match.endpos # position of last !
print match.span()
an example word:cat!!
word:cat
11
19
21
(11, 19)
re.match
re.match和re.search很相似，只是re.match是从字符串的开头开始匹配。
s = 'python tuts'
match = re.match(r'py',s)
if match:
print(match.group())
py
s = 'python tuts'
match = re.search(r'^py',s)
if match:
print(match.group())
py
常用正则字符意义
-
a, X, 9,等字符匹配自己, 元字符不匹配自己，因为有特殊意义，比如** . ^ $ * + ? { }[ ] \ | ( )**
-
. 英文句号，匹配任意字符，不包含'\n'
-
\w 匹配'word'字符，[a-zA-Z0-9]
-
\W 匹配非'word'字符
-
\b 匹配'word'和'non-word'之间边界
-
\s 匹配单个whitespace字符,space, newline, return, tab, form [\n\r\t\f]
-
\S 匹配non-whitespace字符
-
\t, \n, \r 匹配tab, newline, return
-
\d 匹配数字[0-9]
-
^ 匹配字符串开头
-
$ 匹配字符串结尾
重复
‘+’ 一或多次, ‘*’ 零或多次, ‘?’ 零或一次
方括号[]
# 提取email address
string = 'purple alice-b@gmail.com monkey dishwasher'
match = re.search(r'\w+@\w+', string)
if match:
print match.group() ## 'b@google'
b@gmail
[]类似于or
Square brackets can be used to indicate a set of chars, so [abc] matches 'a' or 'b' or 'c'.
match = re.search(r'[\w.-]+@[\w.-]+',string)
if match:
print match.group()
alice-b@gmail.com
Group Extraction圆括号()
有时候需要提取匹配字符的一部分，比如刚才的邮箱，我们可能需要其中的username和hostname，这时候可以用()分别把username和hostname包起来，就像r'([\w.-]+)@([\w.-]+)',如果匹配成功，那么pattern不改变，只是可以用match.group(1)和match.group(2)来username和hostname，match.group()结果不变。
string = 'purple alice-b@google.com monkey dishwasher'
match = re.search(r'([\w\.-]+)@([\w\.-]+)',string)
if match:
# Return subgroup(s) of the match by indices or names.
print match.group() # or match.group(0)
print match.group(1)
print match.group(2)
if match:
# Return a tuple containing all the subgroups of the match, from 1.
print match.groups()
alice-b@google.com
alice-b
google.com
('alice-b', 'google.com')
findall and groups
()和findall()结合，如果包括一或多个group，就返回a list of tuples。
str = 'purple alice@google.com, blah monkey bob@abc.com blah dishwasher'
tuples = re.findall(r'([\w\.-]+)@([\w\.-]+)', str)
print tuples # [('alice', 'google.com'), ('bob', 'abc.com')]
for tuple in tuples:
print tuple[0] # username
print tuple[1] # host
[('alice', 'google.com'), ('bob', 'abc.com')]
alice
google.com
bob
abc.com
给re.search加^之后是一样的。
re.sub
re.sub(pat, replacement, str)在str里寻找和pattern匹配的字符串，然后用replacement替换。replacement可以包含\1或者\2来代替相应的group，然后实现局部替换。
# replace hostname
str = 'alice@google.com, and bob@abc.com'
#returns new string with all replacements,
# \1 is group(1), \2 group(2) in the replacement
print re.sub(r'([\w\.-]+)@([\w\.-]+)', r'\1@yo-yo-dyne.com', str)
alice@yo-yo-dyne.com, and bob@yo-yo-dyne.com
