Summarize and organize Python regular expression knowledge points

This article brings you relevant knowledge about python, which mainly introduces issues related to regular expressions. A regular expression (Regular Expression) is a string that can represent Let’s take a look at a piece of regular information. I hope it will be helpful to everyone.

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1.1 Regular expression

Regular expression An expression (Regular Expression) is a string, it can represent a regular piece of information. Python comes with a regular expression module, through which you can find, extract, and replace a regular piece of information. It is difficult to find one person among ten thousand people, but it is easy to find one very "characteristic" person among ten thousand people. Suppose there is a person with green skin and a height of three meters. Even if this person is among ten thousand people, others can find him at a glance. This "searching" process is called "matching" in regular expressions. In program development, if you want a computer program to find the required content from a large piece of text, you can use regular expressions. There are following steps to use regular expressions.
(1) Look for patterns.
(2) Use regular symbols to express rules.
(3) Extract information.

1.2 Basic symbols of regular expressions

1.2.1 Dot “.”

A dot can replace any a character except the newline character, including but not limited to English letters, numbers, Chinese characters, English punctuation marks and Chinese punctuation marks.

1.2.2 Asterisk “ * ”

An asterisk can represent the subexpression in front of it (ordinary characters, another or several regular expression symbols) 0 times to unlimited times.

All of the above are acceptable: (the asterisk represents the previous expression)

1.2.3 Dot asterisk ".* ”

The dot represents any non-newline character, and the asterisk represents matching the character before it 0 times or any number of times. So ".*" means matching a string of any length any number of times.

All of the above are acceptable:
It means that "any number of any characters other than newline characters" appear between "such" and "ha".

1.2.4 Question mark “? ”

The question mark represents 0 or 1 times of the subexpression before it. Note that the question mark here is the English question mark
. All of the above are acceptable:

1.2.5 Dot and asterisk question mark “.*?” (most commonly used)

Usage after combination:

All of the above can be used:

Note: The difference between ".*?" and ".*"
. *? It means to match the shortest string that can meet the requirements.
One sentence summary is as follows.
① ".*": Greedy mode, obtain the longest string that meets the conditions.
② ".*? ": Non-greedy mode, obtain the shortest string that meets the conditions.

1.2.6 Parentheses "()"

"Extract" part of the content from a string.
There is the following string:

As you can see, the password here has an English colon on the left and a Chinese character "you" on the right. When constructing a regular expression: .*? When you, the result will be:

However, the colon and the Chinese character "you" are not part of the password. If you only want "12345abcde", you need to use brackets:

get:

1.2.7 Backslash “ \ ”

In regular expressions, many symbols have special meanings, such as question marks, asterisks, braces, square brackets and parentheses. Backslash needs to be used in conjunction with other characters to turn special symbols into ordinary symbols, and ordinary symbols into special symbols.

1.2.8 Number “\d”

Use “\d” in regular expressions to represent one digit.
If you want to extract two numbers, you can use \d\d; if you want to extract three numbers, you can use \d\d\d. But what if you don’t know how many digits this number has? You need to use the * sign to represent an arbitrary number.

All can be represented by the following regular expression:

1.3 Using regular expressions

The name of Python's regular expression module is "re", which is the abbreviation of "regular expression". In Python, you need to import this module first before using it. The imported statement is:

import re

1.3.1 findall method

Python’s regular expression module contains a findall method, which can return all items that meet the requirements in the form of a list String.
The function prototype of findall is:

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

pattern represents a regular expression, string represents the original string, and flags represents some special function flags. The result of findall is a list containing all matching results. If no results are matched, an empty list will be returned.

When you need to extract certain content, use parentheses to enclose the content so that you will not get irrelevant information. How to return if it contains multiple "(.*? )"? As shown in Figure 3-2, what is returned is still a list, but the elements in the list become tuples. The first element in the tuple is the account number, and the second element is the password.

There is a flags parameter in the function prototype. This parameter can be omitted. When not omitted, it has some auxiliary functions, such as ignoring case, ignoring newlines, etc.
Here we take ignoring newlines as an example to illustrate. To ignore newlines, you need to use the "re.S" flag.
Although the symbol "\n" appears in the matched results, it is better than getting nothing. The line breaks in the content can be replaced when cleaning the data later.

1.3.2 search method

The usage of search() is the same as that of findall(), but search() will only return the first string that meets the requirements. Once it finds something that matches the requirements, it stops looking. It is especially useful for finding only the first data from a super large text, which can greatly improve the running efficiency of the program.

The function prototype of search() is:
For the result, if the match is successful, it is a regular expression object; if no data is matched, it is None.

If you need to get the matching result, you need to use the .group() method to get the value inside.

Only when the parameter in .group() is 1, will the result in the brackets in the regular expression be printed.

.group() parameters cannot exceed the number of parentheses in the regular expression. A parameter of 1 means reading the content in the first bracket, a parameter of 2 means reading the content in the second bracket, and so on.
(Note that the one in the picture is not findall)

1.3.3 compile method

re.findall() comes with the function of re.compile(), so There is no need to use re.compile().

1.4 Regular expression extraction techniques

1.4.1 First grasp the big ones and then the small ones: secondary extraction

1.4.2 Inside the brackets Outside the brackets

There can be other characters inside the brackets.
See the figure below for specific impacts.

If there are other ordinary characters in the brackets, these ordinary characters will appear in the obtained results.

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