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Editor to share with you what Python regular expressions are, I believe most people do not know much about it, so share this article for your reference, I hope you can learn a lot after reading this article, let's learn about it!

1 preface

A regular expression is a logical formula for manipulating strings (including ordinary characters (for example, letters between an and z) and special characters (called "metacharacters"). It is to use predefined specific characters and a combination of these specific characters to form a "regular string", which is used to express a filtering logic for a string. A regular expression is a text pattern that describes one or more strings to match when searching for text.

All of the above are official instructions, and the blogger's own understanding is (for reference only): by specifying the matching rules for some special characters in advance, and then using these characters to match a variety of complex string scenarios. For example, today's crawlers and data analysis, string checking and so on all need to use regular expressions to process data.

The regular expression of python is the re module:

The re module enables the Python language to have all the regular expression functions.

The re module also provides functions that are exactly the same as those of these methods, which take a pattern string as their first argument.

2 basic grammar

2.1 match function

Pattern is matched only from the beginning of the string. Here is the syntax of the function:

Re.match (pattern, string, flags = 0)

Here is the description of the parameters:

Pattern-this is the regular expression to match.

String-this is a string that will be searched for patterns that match the beginning of the string.

Flags-you can specify different flags using bitwise OR (|). These are modifiers, as listed in the following table.

The re.match function returns the matching object on success and None on failure. Use the match (num) or groups () function to match the object to get the matching expression.

Example

# not matched from the initial position None import re line ='i can speak good english' matchObj = re.match (r'\ s (\ w*)\ s (\ w*). *', line) if matchObj: print ('matchObj.group ():', matchObj.group ()) print ('matchObj.group ():', matchObj.group (1)) print ('matchObj.group ():', matchObj.group (2)) print ('matchObj.group ():' MatchObj.group (3) else: print ('no matchmakers')

# match import re line ='i can speak good english' matchObj = re.match (r'(I)\ s (\ w*)\ s (\ w*). *', line) if matchObj: print ('matchObj.group ():', matchObj.group ()) print ('matchObj.group ():', matchObj.group (1)) print ('matchObj.group ():' MatchObj.group (2)) print ('matchObj.group ():', matchObj.group (3)) else: print ('no matchmakers')

2.2 search function

It works the same way as match (), but instead of matching from the beginning, search () looks for the first match from anywhere. Here is the syntax for this function:

Re.match (pattern, string, flags = 0)

Here is the description of the parameters:

Pattern-this is the regular expression to match.

String-this is a string that will be searched for patterns that match the beginning of the string.

Flags-you can specify different flags using bitwise OR (|). These are modifiers, as listed in the following table.

The re.search function returns a matching object on success, otherwise it returns None. Use the group (num) or groups () function of the match object to get the matching expression.

Example

Import re line ='i can speak good english' matchObj = re.search ('(. *), line) if matchObj: print ('matchObj.group ():', matchObj.group ()) print ('matchObj.group ():', matchObj.group (1)) print ('matchObj.group ():', matchObj.group (2)) print ('matchObj.group ():' MatchObj.group (3) else: print ('no matchmakers')

2.3 sub function

One of the most important modules that use regular expressions re is sub.

Re.sub (pattern, repl, string, max=0)

This method replaces all strings that appear in RE mode with repl, and replaces all occurrences unless max is provided. This method returns the modified string.

Example

Import re line ='i can speak good english' speak = re.sub (ringing canned recordings notorious line) print (speak) speak1 = re.sub (r'\ s precepts precinct line) # replace all spaces print (speak1)

3 special class grammar

3.1 character class

3.2 Special character classes

3.3 repeated matching

3.4 non-greedy repetition

This matches the minimum number of repeats:

3.5 parenthesis grouping

3.6 backreference

Match the previously matched group again

3.7 Anchor point

You need to specify a matching location.

3.8 Special syntax with parentheses

These are all the contents of the article "what are Python regular expressions?" Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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