regular expression

Overall overview

example	describe
[Pp]ython	matching “Python” or “python”.
rub[ye]	matching “ruby” or “rube”.
[abcdef]	Match any letter in bracket .
[0-9]	Match any number . Be similar to [0123456789].
[a-z]	Match any lowercase letter .
[A-Z]	Match any capital letters .
[a-zA-Z0-9]	Match any letters and numbers .
[^au]	except au All characters except letters .
[^0-9]	Match characters other than numbers .

example	describe
.	Matching elimination “\n” Any single character other than . To match includes ‘\n’ Any character inside , Please use something like ‘[.\n]’ The pattern of .
?	Match a character zero times or once , Another function is the non greedy model
+	matching 1 Times or times
*	matching 0 Times or times
\b	Match a length of 0 The string of
\d	Matches a numeric character . Equivalent to [0-9].
\D	Matches a non-numeric character . Equivalent to [^0-9].
\s	Matches any whitespace characters , Including Spaces 、 tabs 、 Page breaks and so on . Equivalent to [ \f\n\r\t\v].
\S	Matches any non-whitespace characters . Equivalent to [^ \f\n\r\t\v].
\w	Match any word character that includes an underline . Equivalent to ’[A-Za-z0-9_]’.
\W	Match any non word character . Equivalent to ‘[^A-Za-z0-9_]‘.

Character matching

Character set

Character set [] Allow matching a set Probably A single character that appears , We need to match multiple characters closely connected , No symbol separation is required .

Be careful ： Only one character is matched in the character group , If you have determined the characters to match, you don't need to use character groups .

[Pp]ython
 You can choose 
python
Python

Section

Section - Allows us to match characters in a specified interval , Reduce the number of our writing .

1. To match any number, you can use [0-9];
2. If you want to match all lowercase letters , It can be written. [a-z];
3. To match all capital letters, write [A-Z];
4. If you want to match from 5 To 8 The number of can be written as [5-8].

Match special character

Sometimes the characters we want to match happen to have a specific meaning in regular expressions , At this time, we add \ Transference .

For example, we want to match characters -, But characters - Represents an interval in a character group , Then we can use escape character \ To paraphrase .

 Match all characters  -
[\-]

Take the opposite

Reverse sign ^ The negative symbol is only represented in the character group , Indicates that the character group is not taken ^ Words listed after .

such as [^123] Represents a match except 1,2,3 All characters except .

Shortcut matching

Match blanks

\s Represents a match Blank character , Including spaces 、tab、 Line break, etc

Match numbers and letters

\w It means to match any letter , Include case , Equate to [a-zA-Z].

\d Means to match any number , Equate to [0-9].

Word boundaries

Sometimes we just want to match a single word , At this time, we need to match word boundaries \b Symbol

For example, we have sentences i play on playground, If we use it directly play Pairs of words play Match , Will put words playground Medium play Also match in .

At this time , We need to use word boundaries , To match \bplay\b You can match a single play word .

The shortcut is reversed

You only need capital letters to reverse the shortcut .

such as \W、\D、\S、\bplay\B（ It refers to extracting non word boundaries play） etc.

Start and end

In regular expressions Outside the character group ^ Specifies the beginning of a string ,$ Specifies the end of a string .

For example, here are two sentences

python is my favourite
this code in python

^python Means to extract with python Starting string .

python$ Means to extract with python a null-terminated string .

Any character

. Characters represent matches to any single character , it Can only appear outside the square bracket character group .

Optional characters

Sometimes , We may want to match different ways of writing a word , such as color and colour, perhaps honor And honour.

We can use it at this time ? The symbol specifies a character 、 Character groups or other basic units are optional , This means that the regular expression engine will expect this character to appear Zero or one .

such as honou?ru? Express u It's optional , It can appear or not , What can be matched is honor and honour .

If it is .? Represents any optional character , May or may not appear .

Match multiple characters

repeat

Add {N} Represents the number of times this character is repeated .

such as \d{4}, Express \d Repeat it four times , Equate to \d\d\d\d, Means to match a four digit number .

Repetition interval

Maybe sometimes , We don't know how many times to match the character group , For example, the ID card has 15 There are also 18 Bit .

Repeat the interval here and you can play , grammar ：{M,N},M Is the lower bound and N It's the upper bound .

For example, we need to match three digits , Four and five digits , You can use it \d{3,5} To match .

\d{3,4} Can match 3 A number can also match 4 A digital , But when there is 4 A number , The priority match is 4 A digital , This is because regular expressions default to Greedy mode , That is, match as many characters as possible , And you want to use Non greedy model , We will be having Add... After the expression ? Number .

Open close interval

{1,} Indicates that the repetition of characters has no boundary , It means matching one or countless .

+ Equate to {1,}

* Equate to {0,}

grouping

Group extraction

In regular expressions, there is also a way to grouping The mechanism of , When using grouping , In addition to getting the whole match . You can also select each group in the match .

Grouping is simple , Use () that will do .

For example, I have to extract <div>hello</div> Medium hello , We can use <div>(.*?)</div>

Or conditions

While using grouping, you can also use perhaps （or） Conditions .

For example, to extract the suffix of all picture files , You can add a | Symbol .

such as (.jpg|.gif|.png)

Non capture grouping

occasionally , We don't need to capture the content of a group , But I want to use the characteristics of grouping .

At this time, you can use the non capture group (?: expression ), thus Do not capture data , You can also use the function of grouping .

such as 0731-75855, We just extract 75855, It can be used (?:\d{4})-(\d{5})

Group backtracking

Backtracking reference of group , Use \N The reference number is N The grouping .

For example, we have <div>hello</div>, We can use <(\w+)>(hello)</\1>, Inside \1 It stands for , The value in the first grouping in the expression .

Greedy mode and non greedy mode

Greedy mode ： Regular expressions tend to match the maximum length , Always match as many characters as possible .

Non greedy model ： Regular expressions tend to match the shortest length , Always match as few characters as possible .

For example, there is a string abcdefabc

Here are regular expressions a.*c

The matching result in greedy mode is ：abcdefabc

In the non greedy mode, the matching result is ：abc

In regular expressions , By default, greedy mode , If we want to use non greedy mode, we need to add a after the quantifier ?, such as a.*?c