1、查找文本中的模式
search()函数取模式和要扫描的文本作为输入，找到这个模式时就返回一个match对象。如果没有找到模式，search()就返回None。
每个match对象包含有关匹配性质的信息，包含原输入字符串，所使用的正则表达式以及模式在原字符串出现的位置。
import re
pattern = 'this'
text = 'Does this text match the pattern?'
match = re.search(pattern, text)
s = match.start()
e = match.end()
print('Found "{}"\nin "{}"\nfrom {} to ("{}")'.format(match.re.pattern, match.string, s, e, text[s:e]))
_____________________输出___________________________________________
Found "this"
in "Does this text match the pattern?"
from 5 to ("9")start()和end()方法可以提供字符串中的相应索引，指示与模式匹配的文本在字符串中出现的位置。
2、编译表达式
尽管re包括模块级函数，可以处理作为文本字符串的正则表达式，但是对于程序频繁使用的表达式而言，编译它们会更为高效。compile()函数会把一个表达式字符串转换为一个Regex0bject。
import re
regexes = [
re.compile(p)
for p in ['this', 'that']
]
text = 'Does this text match the pattern?'
print('Text: {!r}\n'.format(text))
for regex in regexes:
print('Seeking "{}" ->'.format(regex.pattern),end=' ')
if regex.search(text):
print('match')
else:
print('no match')
_________________________输出_________________________________
Text: 'Does this text match the pattern?'
Seeking "this" -> match
Seeking "that" -> no match
模块级函数会维护一个包含已编译表达式的缓存，不过这个缓存的大小是有限的，另外直接使用已编译表达式可以避免与缓存查找相关的开销。使用已编译表达式的另一个好处为，通过在加载模块时预编译所有的表达式，可以把编译工作转移到应用开始时，而不是当程序响应一个用户动作时才编译。
3、多重匹配
使用search()来查找字面量文本字符串的单个实例，findall()函数会返回输入中与模式匹配而且不重叠的所有子串。
import re
text = 'abbaaabbbbaaaaaa'
pattern = 'ab'
for match in re.findall(pattern, text):
print('Found {!r}'.format(match))
_______________________输出_____________________
Found 'ab'
Found 'ab'
finditer()返回一个迭代器，它会生成Match实例，而不是像findall()那样返回字符串。
import re
text = 'abbaaabbbbaaaaaa'
pattern = 'ab'
for match in re.finditer(pattern, text):
s = match.start()
e = match.end()
print('Found {!r} at {:d}:{:d}'.format(text[s:e],s,e))
_______________________输出_______________________________
Found 'ab' at 0:2
Found 'ab' at 5:7
4、模式语法
正则表达式还支持更加强大的模式。模式可以重复，可以锚定到输入中不同的逻辑位置，可以用紧凑的形式表述而不需要在模式中提取每一个重复的字符
import re
def test_pattern(text,patterns):
for pattern, desc in patterns:
print("'{}' ({})\n".format(pattern,desc))
print(" '{}'".format(text))
for match in re.finditer(pattern, text):
s = match.start()
e = match.end()
substr = text[s:e]
n_backslashes = text[:s].count('\\')
prefix = '.' * (s+n_backslashes)
print(" {}'{}'".format(prefix,substr))
print()
return
if __name__ == "__main__":
test_pattern('abbaaabbbbaaaaaa',[('ab',"'a' follow by 'b'"),])
________________________输出_______________________________________
'ab' ('a' follow by 'b')
'abbaaabbbbaaaaaa'
'ab'
.....'ab'
输出显示输入文本以及输入中与模式匹配的各个部分的子串区间。
重复
模式中有5种表示重复的方法。模式后面如果有元字符*，则表示重复0次或多次（允许一个模式重复0次是指这个模式即使不出现也可以匹配）。如果把*替换为+，那么模式必须至少出现一次才能匹配。使用？表示模式出现0次或1次。如果要制定出现次数，需要在模式后面使用{m}，m表示模式应重复的次数。最后，如果要允许一个可变但有限的重复次数，那么可以使用{m,n}，这里m是最少重复次数，n是最大重复次数。如果省略n({m,}),则表示值必须至少出现m次，但没有最大限制。
test_pattern('abbaabbba',[('ab*','a followed by zero or more b'),
('ab+','a followed by one or more b'),
('ab?','a followed by zero or one b'),
('ab{3}','a followed by three b'),
('ab{2,3}','a followed by two or three b')],
)
__________________________输出__________________________________
'ab*' (a followed by zero or more b)
'abbaabbba'
'abb'
...'a'
....'abbb'
........'a'
'ab+' (a followed by one or more b)
'abbaabbba'
'abb'
....'abbb'
'ab?' (a followed by zero or one b)
'abbaabbba'
'ab'
...'a'
....'ab'
........'a'
'ab{3}' (a followed by three b)
'abbaabbba'
....'abbb'
'ab{2,3}' (a followed by two or three b)
'abbaabbba'
'abb'
....'abbb'
处理重复指令时，re在匹配模式时通常会尽可能多地消费输入。这种像“贪心”的行为可能会导致单个匹配减少，或匹配结果包含比预想更多的输入文本。可以在重复指令后面加？来关闭贪心行为。
test_pattern('abbaabbba',[('ab*?','a followed by zero or more b'),
('ab+?','a followed by one or more b'),
('ab??','a followed by zero or one b'),
('ab{3}?','a followed by three b'),
('ab{2,3}?','a followed by two or three b')],
)
______________________________输出________________________________
'ab*?' (a followed by zero or more b)
'abbaabbba'
'a'
...'a'
....'a'
........'a'
'ab+?' (a followed by one or more b)
'abbaabbba'
'ab'
....'ab'
'ab??' (a followed by zero or one b)
'abbaabbba'
'a'
...'a'
....'a'
........'a'
'ab{3}?' (a followed by three b)
'abbaabbba'
....'abbb'
'ab{2,3}?' (a followed by two or three b)
'abbaabbba'
'abb'
....'abb'
字符集
字符集是一组字符，包含可以与模式中当前位置匹配的所有字符。例如，[a,b]可以匹配为a或b
test_pattern('abbaabbba',[('[ab]','either a or b'),
('a[ab]+','a followed by one or more a or b'),
('a[ab]+?','a followed by one or more a or b, not greedy')],
)
________________________________输出__________________________________________
'[ab]' (either a or b)
'abbaabbba'
'a'
.'b'
..'b'
...'a'
....'a'
.....'b'
......'b'
.......'b'
........'a'
'a[ab]+' (a followed by one or more a or b)
'abbaabbba'
'abbaabbba'
'a[ab]+?' (a followed by one or more a or b, not greedy)
'abbaabbba'
'ab'
...'aa'
尖字符（^）代表要查找不在这个尖字符后面的集合中的字符
test_pattern('This is some text -- with punctuation',[('[^-. ]+','sequences without -, ., or space')],)
__________________________________输出__________________________________
'[^-. ]+' (sequences without -, ., or space)
'This is some text -- with punctuation'
'This'
.....'is'
........'some'
.............'text'
.....................'with'
..........................'punctuation'
利用字符区间来定义一个字符集
test_pattern('This is some text -- with punctuation',
[('[a-z]+','sequences of lowercase letters'),
('[A-Z]+','sequences of uppercase letters'),
('[a-zA-Z]+','sequences of lower- or uppercase letters'),
('[a-z][A-Z]+','one uppercase followed by lowercase')],
)
————————————————————————————————————输出————————————————————————————————————
'[a-z]+' (sequences of lowercase letters)
'This is some text -- with punctuation'
.'his'
.....'is'
........'some'
.............'text'
.....................'with'
..........................'punctuation'
'[A-Z]+' (sequences of uppercase letters)
'This is some text -- with punctuation'
'T'
'[a-zA-Z]+' (sequencesof lower- or uppercase letters)
'This is some text -- with punctuation'
'This'
.....'is'
........'some'
.............'text'
.....................'with'
..........................'punctuation'
'[a-z][A-Z]+' (one uppercase followed by lowercase)
'This is some text -- with punctuation'
元字符点号（.）指示模式应当匹配该位置的单个字符
test_pattern('This is some text -- with punctuation',
[('a.','a followed by any one character'),
('b.','b follwed by any one character'),
('a.*b','a followed by anything ending in b'),
('a.*?b','a followed by anything, ending in b')],
)
____________________________输出_____________________________________
'a.' (a followed by any one character)
'This is some text -- with punctuation'
................................'at'
'b.' (b follwed by any one character)
'This is some text -- with punctuation'
'a.*b' (a followed by anything ending in b)
'This is some text -- with punctuation'
'a.*?b' (a followed by anything, ending in b)
'This is some text -- with punctuation'
转义码
| 转义码 | 含义 |
| \d |
数字 |
| \D |
非数字 |
| \s | 空白符（制表符、空格、换行等） |
| \S | 非空白符 |
| \w |
字母数字 |
| \W | 非字母数字 |
test_pattern('A prime #1 example!',
[(r'\d+','sequence of digits'),
(r'\D+','sequence of non-digits'),
(r'\s+','sequence of whitespqce'),
(r'\S+','sequence of non-whitespqce'),
(r'\w+','alphanumeric characters'),
(r'\W+','non-alphanumeric'],
)
___________________________输出_______________________________
'\d+' (sequence of digits)
'A prime #1 example!'
.........'1'
'\D+' (sequence of non-digits)
'A prime #1 example!'
'A prime #'
..........' example!'
'\s+' (sequence of whitespqce)
'A prime #1 example!'
.' '
.......' '
..........' '
'\S+' (sequence of non-whitespqce)
'A prime #1 example!'
'A'
..'prime'
........'#1'
...........'example!'
'\w+' (alphanumeric characters)
'A prime #1 example!'
'A'
..'prime'
.........'1'
...........'example'
'\W+' (non-alphanumeric)
'A prime #1 example!'
.' '
.......' #'
..........' '
..................'!'
要匹配正则表达式语法中包含的字符，需要转义搜索模式中的字符。
test_pattern(r'\d+ \D+ \s+',[(r'\\.\+','escape code')],)
________________________输出_____________________________
'\\.\+' (escape code)
'\d+ \D+ \s+'
'\d+'
.....'\D+'
..........'\s+'
锚定
可以使用锚定指令指定模式在输入文本中的相对位置。
正则表达式锚定码
| 锚定码 | 含义 |
| ^ | 字符串或行开头 |
| $ | 字符串或行末尾 |
| \A | 字符串开头 |
| \Z | 字符串末尾 |
| \b | 单词开头或末尾的空串 |
| \B | 不在的单词开头或末尾的空串 |
test_pattern('This is some text -- with punctuation',
[(r'^\w+','word at start of string'),
(r'\A\w+','word at start of string'),
(r'\w+\S*$','word near end of string'),
(r'\w+\S*\Z','word near end of string'),
(r'\bt\w+','t at end of word'),
(r'\Bt\B','not start or end of word')],
)
_____________________________输出__________________________________
'^\w+' (word at start of string)
'This is some text -- with punctuation'
'This'
'\A\w+' (word at start of string)
'This is some text -- with punctuation'
'This'
'\w+\S*$' (word near end of string)
'This is some text -- with punctuation'
..........................'punctuation'
'\w+\S*\Z' (word near end of string)
'This is some text -- with punctuation'
..........................'punctuation'
'\bt\w+' (t at end of word)
'This is some text -- with punctuation'
.............'text'
'\Bt\B' (not start or end of word)
'This is some text -- with punctuation'
.......................'t'
..............................'t'
.................................'t'