Regular Expression BASICS (Reading Notes), regular expression Reading Notes

Regular Expression BASICS (Reading Notes), regular expression Reading Notes

A regular expression (regex) is a tool.

Two basic functions of a regular expression: search and replace.

 

. Character (English period) can match any single character. (. Characters can match any single character, number, letter, or even. character itself .)

\ Escape character. This is a metacharacter, which indicates that this character has a special meaning rather than its own meaning .)

 

(Conclusion:. can match any character; \ is used to escape the character .)

 

[And] do not match any characters. They are only responsible for defining a character set combination.

 

-A hyphen (-) is a metacharacters that can be used to define a character range. As a metacharacters, it can only be used between [and]. It is only a common character except for a desirable character set.

 

Valid character range:

A-Z, matching all the uppercase letters from A to Z;

A-z: matches all lowercase letters from a to z;

A-z: matches all letters from ASCII character A to ASCII letter z (not commonly used );

 

^ Non-characters are also metacharacters used to perform non-operations on a character set combination.

 

Metacharacters can be roughly divided into two types: one is used to match text (for example,.), and the other is required by the regular expression syntax (for example, [and]).

 

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Blank metacharacters:

[\ B]	Roll back (and delete) one character (Backspace key)
\ F	Page feed
\ N	Line Break
\ R	Carriage Return
\ T	Tab (Tab key)
\ V	Vertical Tab

 

Numeric metacharacters:

\ D	Any numeric character (equivalent to [0-9])
\ D	Any non-numeric character (equivalent to [^ 0-9])

 

Alphanumeric metacharacters:

\ W	Any letter, digit (case-sensitive) or underscore (equivalent to [a-zA-Z0-9 _])
\ W	Any non-alphanumeric or non-underline character (equivalent to [^ a-zA-Z0-9 _])

 

Blank metacharacters:

\ S	Any blank character (equivalent to [\ f \ n \ r \ t \ v])
\ S	Any non-blank character (equivalent to [^ \ f \ n \ r \ t \ v])

 

+ Match one or more characters at a time or multiple times (at least one character does not match zero characters ).

 

* Matches one or more characters zero or multiple times.

 

? Matches zero or one occurrence of one or more characters.

 

{N} sets an exact value for the number of repeated matches (for example, {3} indicates that the previous character or character set must appear three times in a row ).

 

{N, m} sets an interval for the number of repeated matches (for example, {2, 4} indicates that the previous character or character set combination appears at least twice consecutively, at most 4 times, {3 ,} indicates that the previous character or character set must appear at least three times ).

 

Greedy metacharacters and their lazy versions:

*	*?
+	+?
{N ,}	{N ,}?

 

(Conclusion: the real power of a regular expression is reflected in the repeat matching. + One or more occurrences of matching characters or character sets, * zero or multiple occurrences of matching characters or character sets ,? Matches zero or one occurrence of a character or character set. To get more precise control, you can use the {} syntax to precisely control the minimum and maximum values of a repeat or repeat. Metacharacters are classified into two types: "greedy" and "lazy". To prevent over-matching, use the "lazy" metacharacters to construct regular expressions .)

 

\ B is used to match the start or end of a word.

 

\ B is used to match the start or end of a character.

 

^ Defines the start of a string, and $ defines the end of a string.

 

(? M) used to enable the Branch matching mode ,(? M) must appear at the beginning of the entire mode.

 

(Conclusion: regular expressions can be used not only to match text blocks of any length, but also to match text that appears at a specific position of a string. \ B is used to specify a word boundary (\ B is the opposite ). ^ And $ are used to customize the string boundary (the start of a string and the end of a string ). If (? M) in combination, ^ and $ will also match the string starting or ending at the beginning of a line break (at this time, the line break will be considered as a string separator ).)