One article to get regular expressions in Python

Detailed re module###

This article explains in detail regular expressions and the re module in Python

• What is regular expression
• Regular expression function
• Metacharacters and their meaning
• Detailed re module
• Regex modifier
• Regular expression example

<!- - MORE-->

Contents of this article###

What is a regular expression

Regular expression (regular expression) describes a string matching pattern (pattern), can be used to check whether a string contains a certain substring, replace the matched substring, or extract from a string that meets a certain condition Substring etc.

Regular expression function###

By using regular expressions, you can:

1. Test the pattern within the string
   For example, you can test the input string to see if a phone number pattern or a credit card number pattern appears in the string. This is called data verification.
2. Replacement text
   You can use regular expressions to identify specific text in a document, delete the text completely or replace it with other text.
3. Extract substrings from strings based on pattern matching
   You can search for specific text in the document or in the input field, such as the content that you need directly from the content of the webpage through the crawler

Metacharacters and their meaning

Commonly used metacharacters

Metacharacters

The following is a relatively complete metacharacter matching table

Detailed re module###

There module is provided in python to deal with regular expression problems. Here are a few commonly used methods

re.match

re.match tries to match a pattern from the starting position of the string. If the matching is not successful at the starting position, match() returns none.

This method returns a regular matching object

Syntax

import re
re.match(pattern, string, flags=0)

Parameter Description#####

demo

• Get content through group()
• Get the range through span()

# Most regular match
content ="Hello 1234567 World_This is a Regex Demo"print(len(content))
result = re.match("^Hello\s\d+\s\w{10}.*?Demo$", content)   #Must be matched from the starting position
# result = re.match("^Hello\s\d{7}\s\w{10}.*?Demo$", content)print(result)print(result.group())print(result.span())

If there is a newline character, use the flag re.S

# If there is a newline, use the flag symbol

content ="""Hello 1234567 World_This is a Regex Demo.
My name is Peter
I am from shenzhen
"""
print(len(content))
result = re.match("^Hello\s\d+\s.*?shenzhen$", content, re.S)
# result = re.match("^Hello\s\d{7}\s\w{10}.*?Peter$", content)print(result)print(result.group())print(result.span())

line ="Cats are smarter than dogs"

matchObj = re.match( r'(.*) are (.*?) .*?', line, re.M|re.I)if matchObj:print("matchObj.group() : ", matchObj.group())  #All content returned
 print("matchObj.group(1) : ", matchObj.group(1))  #Return to the first()Content in
 print("matchObj.group(2) : ", matchObj.group(2))  #2nd
else:print("No match!!")

re.matchUse as little as possible

re.matchUse as little as possible

re.matchUse as little as possible

re.search

re.search scans the entire string and returns the first successful match, otherwise it returns None. This method does not require starting from the starting position. Once the first content that meets the requirements is found, it will stop searching

You can use the group(num) or groups() matching object function to get the result of the matching expression.

Function syntax

re.search(pattern, string, flags=0)

Parameter Description#####

demo

1. Return the first element that matches successfully
2. The parameters in the group() method cannot exceed the number of parentheses

re.findall

re.findall scans the entire string and returns all eligible elements in the form of list

grammar#####

findall(pattern, string, flags=0)

Parameter Description#####

demo

The result is in list form

If the extracted content contains multiple .*?, then the return is still in the form of a list, but the elements inside become a tuple form

re.sub

Use regular expressions to replace certain content in a string

grammar#####

re.sub(pattern, repl, string, count)

Parameter Description#####

The meanings of the parameters are:

• Regular expression
• Replaced content
• Raw string
• The number of replacements, the default is 0, replace all

demo

sub special processing####

re.sub allows special processing of matching items using functions

Two modes###

Two modes refer to: greedy mode and non-greedy mode

3 Symbols

We often use 3 symbols in regular expressions:

• Dot.: indicates that the match is any character except the newline character
• Question mark?: It means 0 or 1 match
• Asterisk*: means matching 0 or any characters

demo

Explanation####

1. In the non-greedy mode example above, the question mark is used? , Which means non-greedy mode, when it starts to match that aaaacb has met the requirements, the first one is found; then it starts to match again, it matches ab; it matches again adceb
2. In the greedy mode example, the program will find the longest string that meets the requirements
3. In the last example, .? is used, which means that there can only be 0 or 1 elements between ab, so there are only two cases in the result

Regex modifier-optional flag

Regular expressions can contain some optional flag modifiers to control the matching pattern. The modifier is specified as an optional flag. Multiple flags can be specified by bitwise OR (|) them. For example, re.I | re.M is set to I and M flags:

Regular expression example###

Character matching

Character class

Special character class

to sum up###

References

Novice Course-Regular Expression

python-regular expression

Regular expression online test

Python3-regular expression

[ Regular Expression Complete](https://blog.csdn.net/qq_28633249/article/details/77686976?utm_medium=distribute.pc_relevant.none-task-blog-BlogCommendFromMachineLearnPai2-1.nonecase&depth_1-utm_source=distribute.pc_relevant.none- task-blog-BlogCommendFromMachineLearnPai2-1.nonecase)

re module