Regular expression of Python basics

What is a regular expression?

Regular expressions are not just Python patents, but there are regular expressions in almost every other language. Just like Bash has a regular expression command--grep, and personally feel that the regular expression in Python is similar to

Grep-o

Regular expressions are powerful tools for working with strings , although they may not be efficient, but powerful!

The regular expression sets a series of grammars, such as ".  "represents any single character, \d represents a single number ... By combining these rules we can form a pattern! Any string that conforms to this pattern, we think it is matched, otherwise it is illegal to represent this string!

Syntax for regular Expressions:

Single character	Specific instructions
.	Match any character (except for line break \ n)
[]	Match character set, [0-1] for numbers, [A-z] for lowercase letters
\	Escape character, change the original meaning of the subsequent character
[^]	Non -
\d	Same as [0-9] function, indicating numbers
\d	Non-digital, i.e. [^\d]
\s	White space character is:[< space >\t\r\f\n\v]
\s	Non-whitespace characters
\w	Word character [a-za-z0-9]
\w	Non-word character [^\w]

The table above is a single character. When we want to match a letter, we can write this: [A-z]. But what if we were to match two characters?

You may think of it this way: [A-z][a-z] good, in this manner also can. But what if we're going to match the length of 10 or even hundreds of or non-qualifying characters? Then we're going to use a number of words.

Quantity words	Description
*	Match the preceding character any time (0 or unlimited)
+	Match the preceding character one or more times
?	Match the preceding character 0 or 1 times
{M,n}	Matches the preceding character minimum m times, up to N times

When we want to match a string that starts with a, we need to match the boundary:

^	Matches the beginning of the string, matching the beginning of each line in multiline mode
$	Matches the end of the string, matching the end of each line in multiline mode
\a	Match string start only
\z	Match string End only
\b	Matches a word boundary, which is the position between a word and a space. For example, ' er\b ' can match ' er ' in ' never ', but not ' er ' in ' verb '.
\b	Matches a non-word boundary. ' er\b ' can match ' er ' in ' verb ', but cannot match ' er ' in ' Never '

Logical characters and groupings:

|	Similar to or
()	Matches an expression within parentheses, and also represents a group
(? Pname..)	Group, specifying an additional alias in addition to the original number
\ n	The character that references the nth grouped expression to
(? P=name)	The character that the grouping expression that references the alias name matches to

In addition, because Python uses \ as an escape character, and the regular expression needs \ As an escape character, if you need to match the character "\" in the text, you need 4 backslashes "\\\\": the first two and the last two are used to escape the backslash in the programming language respectively. To avoid this complex situation, Python provides us with the R prefix (native string)! All the strings in R ' pattern ' pattern mean the original meaning, that is, \ is \, does not represent the escape character!

Python provides re modules that contain all the functions of regular expressions, so if we want to use regular expressions, we need:

Import RE

Python regular expressions commonly used in 5 kinds of operations:

1 Re.metch ("pattern", "string") : The method matches from the head and returns a match object if the head conforms to pattern Otherwise, none is returned)

ImportReret=re.match ('[A-z]','1SHDKFASDF')Print(ret) Result: NoneImportReret=re.match ('[A-z]','SHDKFASDF')Print(ret) Results:<_sre. Sre_match object; span= (0, 1), match='s'>

This method is commonly used to determine whether a match：
Import re
input_str=' ASHDFJHAKDSF '
If Re.match (' [A-z] ', input_str):
    Print (' ok!! ')
Else
print (' not found!! ')

2 re.search (' Pattern ', ' string '): matches the entire string until a match is found, and if a match is found, a match object is returned and none is returned.

Print(Re.search ('[0-9]{3}','sdfkj123')) Results:<_sre. Sre_match object; Span= (5, 8), match='123'>#This is generally usedifRe.search ('[0-9]{3}','sdfkj123'):    Print('ok!!')Else:    Print('Not found!!')

3 Re.split (): slicing a string

Some students will ask, we do not have str.split () or list.split ... method to slice it, why do you use it?

Take a look at the example:

A='a s D G'Print(A.split (' ') Results: ['a','s','D',"',"',"','g']#when there are multiple contiguous spaces in a string, the result of slicing is so unsatisfactory! a='a s D G'Print(Re.split ('\s+', a)) Results: ['a','s','D','g']

4 Re.findall (): Finds all matches to the character and returns it as a list.

Ret=re.findall ('a','askdjflas')print( Ret
Result: [' A ', ' a ']

5 re.sub (' Pattern ', ' Replace with ', ' string ', count=): Replace the matched character with the number of times you can specify.

Ret=re.findall ('a','askdjflas') Print (ret) results: 1skdjfl1s

Application examples of regular expressions:

Match phone Number:

info='my phone is 18766666666'ret=re.search ('1[358]\d{9} ' , info) Print (Ret.group ()) Results:18766666666

Match IP v4 Address:

info='my phone is 18766666666,my IP is 2.187.5.6'ret=re.search ('(25 [0-5]|2[0-4]\d| [0-1]?\d?\d) (\. ( 25[0-5]|2[0-4]\d| [0-1]?\d?\d)] {3}', info)print(Ret.group ()) Result:2.187.5.6

Regular expression of Python basics