python method to extract string using regular expression

Preface
I won’t go into the basic knowledge of regular expressions. If you are interested, you can click here. There are generally two cases of extraction. One is to extract characters at a single position in the text. String, the other is to extract strings with multiple consecutive positions. Log analysis will encounter this situation, and I will talk about the corresponding methods below.
1. String extraction at a single position
In this case we can use the regular expression (.+?) to extract. For example, for a string "a123b", if we want to extract the value 123 between ab, we can use findall with a regular expression, which will return a list containing all the conditions that match the situation.
The code is as follows:

import re
str = "a123b"
print re.findall(r"a(.+?)b",str)#
输出['123']

1.1 Greedy and non-greedy matching
If we have a string "a123b456b", if we want to match a and the last b All values ​​between instead of the values ​​between a and the first occurrence of b, you can use ? to control regular greedy and non-greedy matching.
The code is as follows:

import re
str = "a123b456b"
print re.findall(r"a(.+?)b", str)
#输出['123']#?控制只匹配0或1个,所以只会输出和最近的b之间的匹配情况
print re.findall(r"a(.+)b", str)
#输出['123b456']
print re.findall(r"a(.*)b", str)
#输出['123b456']

1.2 Multi-line matching
If you want multi-line matching, you need to add the re.S and re.M flags. Add After taking re.S. Will match newline characters, default. Will not match newline characters.
The code is as follows:

str = "a23b\na34b"
re.findall(r"a(\d+)b.+a(\d+)b", str)
#输出[]
#因为不能处理str中间有\n换行的情况
re.findall(r"a(\d+)b.+a(\d+)b", str, re.S)
#s输出[('23', '34')]

After adding re.M, the ^$ mark will match each line. By default, ^ and $ will only match the first line.
The code is as follows:

str = "a23b\na34b"
re.findall(r"^a(\d+)b", str)
#输出['23']
re.findall(r"^a(\d+)b", str, re.M)
#输出['23', '34']

2. Extract strings at multiple consecutive positions
In this case we can use the

(?P<name>…)

regular rule expression to extract. For example, if we have a line of webserver access log:

'192.168.0.1 25/Oct/2012:14:46:34 "GET /api HTTP/1.1" 200 44 "http://abc.com/search" "Mozilla/5.0"'

, and we want to extract all the content in this line of log, we can write multiple

(?P<name>expr)

to extract, and the name can be changed to you For the variable named for the position string, expr can be changed to the regular expression for extracting the position.
The code is as follows:

import re
line ='192.168.0.1 25/Oct/2012:14:46:34 "GET /api HTTP/1.1" 200 44 "http://abc.com/search" 
"Mozilla/5.0"'
reg = re.compile('^(?P<remote_ip>[^ ]*) (?P<date>[^ ]*) "(?P<request>[^"]*)" 
(?P<status>[^ ]*) (?P<size>[^ ]*) "(?P<referrer>[^"]*)" "(?P<user_agent>[^"]*)"')
regMatch = reg.match(line)
linebits = regMatch.groupdict()
print linebits
for k, v in linebits.items() :
 print k+": "+v

The output result is:

status: 200
referrer: 
request: GET /api HTTP/1.1
user_agent: Mozilla/5.0
date: 25/Oct/2012:14:46:34size: 44
remote_ip: 192.168.0.1

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