Regular expressions, as a concept of computer science, are usually used to retrieve and replace text that conforms to certain rules. A regular expression is a logical formula for manipulating strings. It uses predefined regular strings to filter strings.

Regular expressions are essentially a small, highly specialized programming language. In Python, regular expressions are implemented through the RE module. A regular expression can specify rules for the corresponding set of strings to be matched, and then modify or delimit strings in some way through the RE module.

The regular expression pattern is compiled into a series of bytecodes and executed by a matching engine written in C, so it is somewhat faster than writing Python string processing code directly. But not all string matching can be done with regular expressions, and even if you can handle single expressions, it can be complicated and unreadable, which is recommended to write Python code directly.

The re module has been added to Python since version 1.5, providing perl-style regular expression patterns. The re module is embedded in Python, so it can be imported directly, using method __version__ to view the version, and method __all__ to view attribute methods:

import re
print(re.__version__)
print(re.__all__)
The output is as follows:
2.21.
['match'.'fullmatch'.'search'.'sub'.'subn'.'split'.'findall'.'finditer'.'compile'.'purge'.'template'.'escape'.'error'.'Pattern'.'Match'.'A'.'I'.'L'.'M'.'S'.'X'.'U'.'ASCII'.'IGNORECASE'.'LOCALE'.'MULTILINE'.'DOTALL'.'VERBOSE'.'UNICODE']
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The above code shows that the RE module does not involve many functions. The function is to find patterns in the text, the second is to compile expressions, and the third is to match multiple layers. At the same time, it also defines some constants.

The search() function is used primarily to find patterns in text. This function has three parameters: pattern, string, and flags.

• Pattern represents the expression string used at compile time
• String indicates the string to be matched
• Flags indicates the compilation flag, which is used to change the matching mode of the regular expression, such as case sensitivity and multi-line matching. The default value is 0. Common flag values are as follows:

re.serach()The function takes a pattern and scanned text as input and returns a match, or None if no pattern is found:



See from thematchContains information about the nature of the match for the returned match object. The position at which the pattern appears in the original string when using a regular expression, hasstart(),end(),group(),span(),groups()Methods:

• start()Returns the starting position of the match
• end()Returns the end of the match
• group()Returns the matched string
• span()Returns a tuple containing the matching (start, end) position
• groups()Returns a tuple containing all of the subgroup strings in the regular expression, from 1 to the contained subgroup number, usually without arguments. In addition, there is anothergroup(n,m)Method to return the string matched by group number (n,m).

Third, compile() precompile

Using the functioncompile()Regular expressions are compiled into regular expression objects to improve execution efficiency. This function returns an object schema withpattern,flags=0Two parameters, meaning and abovesearch()Consistency is mentioned in.



Usually compiled expressions are frequently used by the program, which is more efficient to compile, but also requires some caching overhead. Another advantage of using compiled expressions is that all expressions are compiled as soon as the module is loaded, rather than when the program responds to user action.

四, Match () start position match

Using the functionmatch()Matches at the beginning of the text string. This method is not a complete match, but only matches at the beginning of the string.

Findall () returns a list of all matched strings in a string. This function takes the same effect as the search() function, but returns all matching and non-overlapping substrings.

functionfinditer()How to use andfindall()Same, except that it returns an iterator instead of a list. It will generate Match instances.

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A: We split it up

functionsplit()The ability to split the string to be matched in a substring and return a list.



Sub () and subn()

functionsubReplaces each matching substring in the string with pattern and returns the replaced substring in the formatre.sub(pattern,repl,string,count,flag). The functionsubn()Returns one more number of substitutions on this basis.

• Pattern is an expression string
• Repel is the replaced character
• String is a string used for matching
• Count is the maximum number of substitutions
• Flag with the above

Numeric expression verification is mainly used to match the regular expression correction of the numbers in the text. The following will explain some common expressions and use the RE module to process them.

1. ^ [0-9] * $
   
   And as mentioned earlier^Is the start of the matching string;$Is the end position of the matching character;[0-9]Represents any number;*Matches the preceding character 0 or umpteenth times. The following will not be repeated, not clear, please refer to the above.
   
   In summary, this expression is used to match numbers.
   
   
2. ^[1-9]\d*$Matches non-0 positive integers
   
   
3. ^\d{n}$Matches n digits
   
   
4. ^\d{n,}$At least n digits are matched
   
   
5. ^\d{m,n}$Matches numbers from m to N bits
   
   
6. ^ (1-9] [0-9] * [0-9] {1, 2}? $Matches non-zero numbers with at most two decimal digits
   
   

1. ^\w+(-+.\w)*@\w+([-.]\w+)*\.\w+([-.]\w+)*$Verify the E-mail address
   
   
2. ^1[34589]\\d{9}$Mobile phone number
   
   
3. ^[1-9]\d{5}(18|19|([23]\d))\d{2}((0[1-9])|(10|11|12))(([0-2][1-9])|10|20|30|31)\d{3}[0-9Xx]$Id number
   
   
4. ^[1-9]\d{5}(? ! \d)$The zip code
   
   
5. (? i)^([a-z0-9]+(-[a-z0-9]+)*\.) +[a-z]{2,}$The domain name
   
   

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