Regular Expression (Regular Expression)

Regular Expression
It is a text mode consisting of common characters (such as characters a to z) and special characters (called metacharacters. A regular expression is used as a template to match a character pattern with the searched string.
You can construct a regular expression by adding various components in expression mode between a pair of delimiters, that is,/expression/
Common characters
It consists of all the print and non-print characters that are not explicitly specified as metacharacters. This includes all uppercase and lowercase letter characters, all numbers, all punctuation marks, and some symbols.
Non-printable characters
Character meaning
Cx matches the control characters specified by x. For example, cM matches a Control-M or carriage return character. The value of x must be either a A-Z or a-z. Otherwise, c is treated as an original 'C' character.
F matches a form feed. It is equivalent to x0c and cL.
Match A linefeed. It is equivalent to x0a and cJ.
Match a carriage return. It is equivalent to x0d and cM.
S matches any blank characters, including spaces, tabs, and page breaks. It is equivalent to [fv].
S matches any non-blank characters. It is equivalent to [^ fv].
Match a tab. It is equivalent to x09 and cI.
V matches a vertical tab. It is equivalent to x0b and cK.
Special characters
Special characters are characters with special meanings, such as *. txt. in simple words, they represent the meaning of any string. If you want to find a file with * in the file name, you need to escape *, that is, add one before it. Ls *. txt. Regular expressions have the following special characters.
Special characters
$ Matches the end position of the input string. If the Multiline attribute of the RegExp object is set, $ also matches ''or ''. To match the $ character, use $.
() Mark the start and end positions of a subexpression. Subexpressions can be obtained for future use. To match these characters, use (and ).
* Matches the previous subexpression zero or multiple times. To match * characters, use *.
+ Match the previous subexpression once or multiple times. To match + characters, use +.
. Match any single character except line breaks. To match., use.
[Mark the start of a bracket expression. To match [, use [.
? Match the previous subexpression zero or once, or specify a non-Greedy qualifier. To match? Character, please use ?.
Mark the next character as or a special character, or a literal character, or backward reference, or an octal escape character. For example, 'n' matches the character 'n '. ''Matches the linefeed. The sequence ''matches" ", while '(' matches "(".
^ Matches the start position of the input string. Unless used in the square brackets expression, this character set is not accepted. To match the ^ character itself, use ^.