How to extract strings using regular expressions in python

Preface

I won’t go into the basic knowledge of regular expressions. Those who are interested can click here. There are generally two situations for extraction. One is Extract extracts a string at a single position in the text, and the other is to extract a string at 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 matching situations.

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 all values ​​between a and the last b instead of the value between a and the first occurrence of b, we can use ? to control the 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 , then you need to add re.S and re.M flags. After adding 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 Matches 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 regular expression

(?P<name>…)

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(&#39;^(?P<remote_ip>[^ ]*) (?P<date>[^ ]*) "(?P<request>[^"]*)" 
(?P<status>[^ ]*) (?P<size>[^ ]*) "(?P<referrer>[^"]*)" "(?P<user_agent>[^"]*)"&#39;)
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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