当前位置：网站首页>Regular expression learning notes

Regular expression learning notes

2022-07-02 01:49:00 【Stark Zhang Yu】

What is regular expression

Verify the validity of the data 、 Find the text that meets the requirements, and cut and replace the text .

The real power of regular expressions is that they can find text that conforms to a certain rule .

Metacharacters

1. Special string

. Any character except newline
\d Represents any single number
\w Represents any single number or letter or underscore
\s Represents any single blank character

\D \W \S, The meaning is just the opposite .

2. Blank character

\r A carriage return
\n A newline
\f Page identifier
\t tabs
\v Vertical tabs
\s Any whitespace

3. quantifiers

* representative 0 Times to times
+ 1 Times to times
？ 0 To 1 Time
{m} appear m Time
{m,} At least m Time
{m,n} m To n Time

4. Range

| or , Such as ab|bc representative ab or bc
[…] A commonplace , Any single element in parentheses
[a-z] matching a-z Any single element between
[^…] Take the opposite , Cannot include any single element in parentheses

Greedy matching 、 Non greedy matching and exclusive mode

The characteristic of greedy pattern is to match the maximum length as much as possible
Non greedy patterns will match as short as possible .
Exclusive mode and greedy mode are very similar , Exclusive patterns match as much as possible , If the match fails, it ends , No backtracking , This will save time . The specific method is to add a plus sign after the quantifier (+), for example xy{1,3}+yz

Be careful ： Python and Go None of the standard libraries currently support exclusive mode .

(“.+?”)|\w{2,6}

Grouping and referencing

[ Failed to transfer the external chain picture , The origin station may have anti-theft chain mechanism , It is suggested to save the pictures and upload them directly (img-uAMt23It-1656663240092)(https://note.youdao.com/yws/res/20143/WEBRESOURCEf8297d4f75fd7803bb138a4326c9a4fd)]

Regular as shown in the figure , Put the date and time in parentheses . There are two groups in this regular , Date is 1 individual , Time is the first 2 individual .

1. Don't save subgroups

The function of grouping is that it may be referenced later , But if you don't need references, you can add sub groups without saving , expression (?:xxx), The advantage is that the regular performance will be better , It is also less likely to make mistakes when counting subgroups .

2. Parentheses are nested

[ Failed to transfer the external chain picture , The origin station may have anti-theft chain mechanism , It is suggested to save the pictures and upload them directly (img-Euot1k2Y-1656663240095)(https://note.youdao.com/yws/res/20156/WEBRESOURCEe2aeab108bee9c5908c7e6405867abd8)]

Group references are used in lookups

… A little

Four matching patterns

1. Case insensitive mode （Case-Insensitive）

Case insensitive mode , Regular expressions (?i)cat, In this way, the matching characters are not case sensitive

If we want the result on the previous match , The case is the same as that of the second repetition , What should I do ？ We just need to use parentheses to put modifiers and regular cat Part enclosed , Adding parentheses is equivalent to limiting the scope of action , Let case insensitivity only apply to the contents in this bracket . Regular expressions ((?i)cat)

2. Point number universal distribution mode （Dot All）

In the metacharacter . It can match any element except line feed , Use in single line mode ., Regular expressions (?s).

3. Multi line matching pattern

Expression of multiline pattern (?m)^...|...$, The advantage of this matching is that every line

4. Annotation mode

A little …