python之正則運算式

1,什么是正則運算式？

正則運算式（regular expression）是用來簡潔表達一組字串的運算式，

2,作用是什么？

①表達文本型別的特征， ②同時查找或替換一組字串， ③匹配字串的全部或部分，

3,常用的運算子：

4,正則運算式的一些語法實體

5,經典的正則運算式實體

6,re庫的基本使用

①search(pattern, string, flags=0)

pattern：正則運算式的字串或原生字串表示
string：待匹配字串
flags：正則運算式使用時的控制標記

1 import re
2 match = re.search(r"[1-9]\d{5}", "haha 723300")
3 if match:
4     print(match.group())
5 
6 G:\Project1\venv\Scripts\python.exe G:/Project1/practice/lianxi2.py
7 723300
8 
9 Process finished with exit code 0

②match(pattern,string,flags=0)

需要注意的是 match 函式是從字串開始處開始查找，如果開始處不匹配，則不再繼續尋找，若找到回傳值為 一個 match 物件，找不到時回傳 None

 1 import re
 2 match = re.match(r"[1-9]\d{5}", "haha 723300")
 3 print(type(match))
 4 match = re.match(r"[1-9]\d{5}", "723300 haha")
 5 if match:
 6     print(match.group())
 7 
 8 G:\Project1\venv\Scripts\python.exe G:/Project1/practice/lianxi2.py
 9 <class 'NoneType'>
10 723300
11 
12 Process finished with exit code 0

可見search與match的區別在于：
match要求待匹配的子串必須在字串的起始位置，否則查找不到，而search則無此要求

③findall（pattern，string，flags=0）

 1 import re
 2 c = re.findall(r"[1-9]\d{5}", "haha723300 xixi612203")
 3 print(type(c))
 4 print(c)
 5 
 6 G:\Project1\venv\Scripts\python.exe G:/Project1/practice/lianxi2.py
 7 <class 'list'>
 8 ['723300', '612203']
 9 
10 Process finished with exit code 0

④split(pattern，string，maxsplit=0，flags=0)

maxsplit：最大分割數，剩余部分作為最后一個元素輸出

 1 import re
 2 a = re.split(r"[1-9]\d{5}", "haha723300 xixi612203")
 3 print(type(a))
 4 print(a)
 5 
 6 a = re.split(r"[1-9]\d{5}", "haha723300 xixi612203", maxsplit=1)
 7 print(a)
 8 
 9 str1 = "name: hpl, age: 18"
10 b = re.split(r'\:|\,', str1)
11 print(b)
12 
13 
14 G:\Project1\venv\Scripts\python.exe G:/Project1/practice/lianxi2.py
15 <class 'list'>
16 ['haha', ' xixi', '']
17 ['haha', ' xixi612203']
18 ['name', ' hpl', ' age', ' 18']
19 
20 Process finished with exit code 0

⑤finditer(pattern，string，flags=0)

 1 import re
 2 for m in re.finditer(r"[1-9]\d{5}", "haha723300 xixi612203"):
 3     if m:
 4         print(m.group())
 5 
 6 G:\Project1\venv\Scripts\python.exe G:/Project1/practice/lianxi2.py
 7 723300
 8 612203
 9 
10 Process finished with exit code 0

⑥sub(pattern，repl，string，count=0，flags=0)

repl：替換匹配字串的字串
count：匹配的最大替換次數

1 import re
2 m = re.sub(r"[1-9]\d{5}", "love", "haha723300 xixi612203")
3 if m:
4     print(m)
5 
6 G:\Project1\venv\Scripts\python.exe G:/Project1/practice/lianxi2.py
7 hahalove xixilove
8 
9 Process finished with exit code 0

7,re庫的match物件

屬性：
string 待匹配的文本
re 匹配時使用的pattern物件（正則運算式）
pos 正則運算式搜索文本的開始位置
endpos 正則運算式搜索文本的結束位置

方法：
group() 獲得匹配后的字串
start() 匹配字串在原始字串的開始位置
end() 匹配字串在原始字串的結束位置
span() 回傳（start）…（end）

 1 import re
 2 match = re.search(r"[1-9]\d{5}", "haha723300 xixi612203")
 3 print(match.string)
 4 print(match.re)
 5 print(match.pos)
 6 print(match.endpos)
 7 print(match.group())
 8 print(match.start())
 9 print(match.end())
10 print(match.span())
11 
12 G:\Project1\venv\Scripts\python.exe G:/Project1/practice/lianxi2.py
13 haha723300 xixi612203
14 re.compile('[1-9]\\d{5}')
15 0
16 21
17 723300
18 4
19 10
20 (4, 10)
21 
22 Process finished with exit code 0

8,re庫的貪婪匹配和最小匹配

①re庫默認采用貪婪匹配，即輸出匹配最長的子串

1 import re
2 match = re.search(r'PY.*N','PYANBNCNDN')
3 print(match.group())
4 
5 G:\Project1\venv\Scripts\python.exe G:/Project1/practice/lianxi2.py
6 PYANBNCNDN
7 
8 Process finished with exit code 0

②最小匹配的方法：在擴展運算子后加？

1 import re
2 match = re.search(r'PY.*?N','PYANBNCNDN')
3 print(match.group())
4 
5 G:\Project1\venv\Scripts\python.exe G:/Project1/practice/lianxi2.py
6 PYAN
7 
8 Process finished with exit code 0

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