Regular expression summary

Catalog

1.  Introduction to regular expressions

2. regular expression syntax

2.1 Ordinary character

2.2 Print character

2.3 Special characters

2.4 qualifiers

2.5 Locator

2.6 Selection and backreference

3 Modifiers and metacharacters

3.1 Modifier

3.2 Metacharacters

3.3 priority

4 Matching rules

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1.  Introduction to regular expressions

         Regular expressions (Regular Expression) It's a text pattern , Include normal characters （ for example ,a To z Between the letters ） And special characters （ be called " Metacharacters "）. Regular expressions are described using a single string 、 Match a string that matches a syntax rule .

         Why use regular expressions ？

         Typical search and replace operations require you to provide exact text that matches the expected search results . Although this technique may be sufficient for simple search and replace tasks on static text , But it lacks flexibility , If you use this method to search dynamic text , Even if it's not impossible , At least it will be very difficult .

• Test the pattern within the string

• replace text

• Extract substrings from strings for pattern matching

2. regular expression syntax

2.1 Ordinary character

         Normal characters include all printable and nonprintable characters that are not explicitly specified as metacharacters . This includes all uppercase and lowercase letters 、 All figures 、 All punctuation and some other symbols .

• [A-Z] Represents an interval , Match all capital letters ,[a-z] Means all lowercase letters .

•  [...] Represents a match [...] All characters in , for example [aeiou] Match string "google runoob taobao" All of the e o u a Letter .

• Match except [^...] All the characters of the characters in , for example [^aeiou] Match string "google runoob taobao" In addition to e o u a All the letters of the letter .

• .: Match break （\n、\r） Any single character other than , Equivalent to [^\n\r].

• [\s\S]: Match all .\s Is to match all blanks , Including line breaks ,\S Not blank , Not including line breaks .

• \w: Match the letter 、 Numbers 、 Underline . Equivalent to [A-Za-z0-9_]

2.2 Print character

         Nonprinting characters can also be part of regular expressions . The following table lists escape sequences that represent non print characters

2.3 Special characters

         Special characters , Just some characters with special meanings

2.4 qualifiers

         Qualifiers are used to specify how many times a given component of a regular expression must appear to satisfy a match . Yes * or + or ? or {n} or {n,} or {n,m} common 6 Kind of .

         By means of *、+ or ? Place after qualifier ?, The expression is from " greedy " The expression is converted to " Not greed " Expression or minimum match .

2.5 Locator

• ^： Matches where the input string starts ;

• $： Matches the position of the end of the input string ;

• \b： Matches a word boundary , That is, the position between words and spaces ;

• \B： Non word boundary matching

2.6 Selection and backreference

         Use parentheses () Enclose all the options , Use... Between adjacent options | Separate .

         Adding parentheses to both sides of a regular expression pattern or part of a pattern will cause related matches to be stored in a temporary buffer , Each sub match captured is stored in the order that it appears from left to right in the regular expression pattern

3 Modifiers and metacharacters

3.1 Modifier

Modifier	meaning	describe
i	ignore - Case insensitive	Set matching to case insensitive , Search is case insensitive : A and a There is no difference between .
g	global - The global matching	Find all the matches .
m	multi line - Multi-line matching	Make the boundary character ^ and $ Match the beginning and end of each line , Remember it's multiple lines , Instead of the beginning and end of the entire string .
s	Special character dot . Include line breaks in \n	Dots by default . It's match division newline \n Any character other than , add s After the modifier , . Include line breaks in \n.

3.2 Metacharacters

• +： Match previous subexpression one or more times ;

• \d: Matches a numeric character . Equivalent to [0-9].

• \D: Matches a non-numeric character . Equivalent to [^0-9].

• wait , There are still a lot of it

3.3 priority

         Regular expressions are evaluated from left to right , And follow the order of precedence

4 Matching rules

          Everything starts from the most basic . Pattern , Is the most basic element of a regular expression , They are a set of characters that describe the characteristics of a string . The pattern can be simple , Composed of ordinary strings , It can also be very complicated , Special characters are often used to represent characters in a range 、 Recurring , Or context .

About regular expressions in python Please refer to my other blog ：

python Based on learning -- Strings and regular expressions

Reference resources ：https://www.runoob.com/regexp/regexp-tutorial.html