Python extracts strings using regular expressions

Objective

The basics of regular expressions do not say, interested can click here, extract the general in two cases, one is to extract the text to extract a single position of the string, and the other is to extract a number of consecutive positions of the string. Log analysis will encounter this situation, I will tell you the corresponding method.

One, single-position string extraction

In this case we can use (. +?) This regular expression to extract. For example, a string "a123b", if we want to extract the value between AB 123, you can use the findall with the regular expression, which will return a list containing so the matching situation.

The code is as follows:

?

1234	importrestr = "a123b"printre.findall(r"a(.+?)b",str)#输出[‘123‘]

1.1 Greedy and non-greedy matches

If we have a string "a123b456b", if we want to match all the values between A and the last B instead of the value between a and the first occurrence of B, you can use to control the case of regular greedy and non-greedy matches.

The code is as follows:

?

1234567891011

import restr = "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 to match multiple lines, then you need to add re. S and RE.M flags. Plus re. s rear. Will match line breaks, default. Line breaks are not matched.

The code is as follows:

?

12345678

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‘)]

Plus re. After M, the ^$ flag will match each row, and the default ^ and $ will only match the first row.

The code is as follows:

?

1234567

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‘]

String extraction with multiple positions in a row

In this case we can use (?P<name>…) this regular expression to extract. For example, if we have a line of webserver access logs: ‘192.168.0.1 25/Oct/2012:14:46:34 "GET /api HTTP/1.1" 200 44 "http://abc.com/search" "Mozilla/5.0"‘ We want to extract all the contents of this line of log, and can write multiple (?P<name>expr) to extract, where name can be changed to the variable you named for the location string, and expr changes to the regular of the fetch position.

The code is as follows:

?

12345678910

import reline =‘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 linebitsfor k, v in linebits.items() : print k+": "+v

The result of the output is:

?

123456

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

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Python extracts strings using regular expressions